(1104L) Phylum Rotifera & Phylum Platyhelminthes - cnx.org · OpenStax-CNX module: m63244 1 (1104L) Phylum Rotifera & Phylum Platyhelminthes * Je rey Mahr Based on Superphylum Lophotrchozooa

OpenStax-CNX module: m63244 1

(1104L) Phylum Rotifera & PhylumPlatyhelminthes*

Je�rey Mahr

Based on Superphylum Lophotrochozoa� by

OpenStax

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 4.0�

Abstract

By the end of this section, you will be able to:

• Describe the unique anatomical and morphological features of �atworms, rotifers, Nemertea, mol-lusks, and annelids

• Describe the development of an extracoelomic cavity• Discuss the advantages of true body segmentation• Explain the key features of Platyhelminthes and their importance as parasites• Describe the features of animals classi�ed in phylum Annelida

Animals belonging to superphylum Lophotrochozoa are protostomes, in which the blastopore, or thepoint of involution of the ectoderm or outer germ layer, becomes the mouth opening to the alimentary canal.This is called protostomy or ��rst mouth.� In protostomy, solid groups of cells split from the endoderm orinner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band and thensplits internally to form the coelom; this protostomic coelom is hence termed schizocoelom.

As lophotrochozoans, the organisms in this superphylum possess either a lophophore or trochophorelarvae. The lophophores include groups that are united by the presence of the lophophore, a set of ciliatedtentacles surrounding the mouth. Lophophorata include the �atworms and several other phyla. These cladesare upheld when RNA sequences are compared. Trochophore larvae are characterized by two bands of ciliaaround the body.

The lophotrochozoans are triploblastic and possess an embryonic mesoderm sandwiched between theectoderm and endoderm found in the diploblastic cnidarians. These phyla are also bilaterally symmetrical,meaning that a longitudinal section will divide them into right and left sides that are symmetrical. It alsomeans the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organsin the head of the organism, which is where it �rst encounters its environment.

*Version 1.1: Oct 17, 2016 4:28 am -0500�http://cnx.org/content/m44665/1.8/�http://creativecommons.org/licenses/by/4.0/

http://cnx.org/content/m63244/1.1/


1 Phylum Platyhelminthes

The �atworms are acoelomate organisms that include many free-living and parasitic forms. Most of the�atworms are classi�ed in the superphylum Lophotrochozoa, which also includes the mollusks and annelids.The Platyhelminthes consist of two lineages: the Catenulida and the Rhabditophora. The Catenulida, or"chain worms" is a small clade of just over 100 species. These worms typically reproduce asexually bybudding. However, the o�spring do not fully attach from the parents and, resemble a chain in appearance.All of the remaining �atworms discussed here are part of the Rhabditophora. Many �atworms are parasitic,including important parasites of humans. Flatworms have three embryonic tissue layers that give rise tosurfaces that cover tissues (from ectoderm), internal tissues (from mesoderm), and line the digestive system(from endoderm). The epidermal tissue is a single layer cells or a layer of fused cells (syncytium) that coversa layer of circular muscle above a layer of longitudinal muscle. The mesodermal tissues include mesenchymalcells that contain collagen and support secretory cells that secrete mucus and other materials at the surface.The �atworms are acoelomates, so their bodies are solid between the outer surface and the cavity of thedigestive system.

1.1 Physiological Processes of Flatworms

The free-living species of �atworms are predators or scavengers. Parasitic forms feed on the tissues of theirhosts. Most �atworms, such as the planarian shown in Figure 1, have a gastrovascular cavity rather thana complete digestive system. In such animals, the �mouth� is also used to expel waste materials from thedigestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched.Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis.One group, the cestodes, lacks a digestive system. Flatworms have an excretory system with a network oftubules throughout the body with openings to the environment and nearby �ame cells, whose cilia beat todirect waste �uids concentrated in the tubules out of the body. The system is responsible for the regulationof dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nervecords running the length of the body with connections between them and a large ganglion or concentrationof nerves at the anterior end of the worm, where there may also be a concentration of photosensory andchemosensory cells.

There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on di�u-sion and cell-cell junctions. This necessarily limits the thickness of the body in these organisms, constrainingthem to be ��at� worms.

Most �atworm species are monoecious, and fertilization is typically internal. Asexual reproduction iscommon in some groups.



Figure 1: The planarian is a �atworm that has a gastrovascular cavity with one opening that servesas both mouth and anus. The excretory system is made up of tubules connected to excretory pores onboth sides of the body. The nervous system is composed of two interconnected nerve cords running thelength of the body, with cerebral ganglia and eyespots at the anterior end.

1.2 Diversity of Flatworms

Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda(Figure 2). As discussed above, the relationships among members of these classes is being reassessed, withthe turbellarians in particular now viewed as a paraphyletic group, a group that does not have a singlecommon ancestor.



Figure 2: Phylum Platyhelminthes is divided into four classes. (a) Class Turbellaria includes theBedford's �atworm (Pseudobiceros bedfordi), which is about 8�10 cm in length. (b) The parasitic classMonogenea includes Dactylogyrus spp. Dactylogyrus, commonly called a gill �uke, is about 0.2 mm inlength and has two anchors, indicated by arrows, that it uses to latch onto the gills of host �sh. (c) TheTrematoda class includes Fascioloides magna (right) and Fasciaola hepatica (two specimens of left, alsoknown as the common liver �uke). (d) Class Cestoda includes tapeworms such as this Taenia saginata.T. saginata, which infects both cattle and humans, can reach 4�10 meters in length; the specimen shownhere is about 4 meters. (credit a: modi�cation of work by Jan Derk; credit d: modi�cation of work byCDC)

The class Turbellaria includes mainly free-living, marine species, although some species live in freshwateror moist terrestrial environments. The ventral epidermis of turbellarians is ciliated and facilitates theirlocomotion. Some turbellarians are capable of remarkable feats of regeneration in which they may regrowthe body, even from a small fragment.



The monogeneans are ectoparasites, mostly of �sh, with simple lifecycles that consist of a free-swimminglarva that attaches to a �sh to begin transformation to the parasitic adult form. The parasite has only onehost and that host is usually only one species. The worms may produce enzymes that digest the host tissuesor simply graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the malegametes develop �rst and so cross-fertilization is quite common.

The trematodes, or �ukes, are internal parasites of mollusks and many other groups, including humans.Trematodes have complex lifecycles that involve a primary host in which sexual reproduction occurs, and oneor more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk.Trematodes are responsible for serious human diseases including schistosomiasis, a blood �uke. The diseaseinfects an estimated 200 million people in the tropics, leading to organ damage and chronic symptoms likefatigue. Infection occurs when the human enters the water and a larva, released from the primary snail host,locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cellsbefore reproducing. Many of the eggs are released in feces and �nd their way into a waterway, where theyare able to reinfect the primary snail host.

The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates (Figure 3). Tapeworms livein the intestinal tract of the primary host and remain �xed using a sucker on the anterior end, or scolex, of thetapeworm body. The remaining body of the tapeworm is made up of a long series of units called proglottids,each of which may contain an excretory system with �ame cells, but contain reproductive structures, bothmale and female. Tapeworms do not possess a digestive system; instead, they absorb nutrients from the foodmatter passing them in the host's intestine.

Proglottids are produced at the scolex and gradually migrate to the end of the tapeworm; at this point,they are �mature� and all structures except fertilized eggs have degenerated. Most reproduction occurs bycross-fertilization. The proglottid detaches from the body of the worm and is released into the feces of theorganism. The eggs are eaten by an intermediate host. The juvenile worm infects the intermediate host andtakes up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycleis completed. There are several tapeworm parasites of humans that are transmitted by eating uncooked orpoorly cooked pork, beef, and �sh.



Figure 3: Tapeworm (Taenia spp.) infections occur when humans consume raw or undercooked infectedmeat. (credit: modi�cation of work by CDC)

2 Phylum Rotifera

The rotifers are a microscopic (about 100 µm to 30 mm) group of mostly aquatic organisms that get theirname from the corona, a rotating, wheel-like structure that is covered with cilia at their anterior end (Fig-ure 4). Although their taxonomy is currently in �ux, one treatment places the rotifers in three classes:Bdelloidea, Monogononta, and Seisonidea. The classi�cation of the group is currently under revision, how-ever, as more phylogenetic evidence becomes available. It is possible that the �spiny headed worms� currentlyin phylum Acanthocephala will be incorporated into this group in the future.

The body form of rotifers consists of a head (which contains the corona), a trunk (which contains theorgans), and the foot. Rotifers are typically free-swimming and truly planktonic organisms, but the toes orextensions of the foot can secrete a sticky material forming a holdfast to help them adhere to surfaces. Thehead contains sensory organs in the form of a bi-lobed brain and small eyespots near the corona.



Figure 4: Shown are examples from two of the three classes of rotifer. (a) Species from the classBdelloidea are characterized by a large corona, shown separately from the whole animals in the centerof this scanning electron micrograph. (b) Polyarthra, from the class Monogononta, has a smaller coronathan Bdelloid rotifers, and a single gonad, which give the class its name. (credit a: modi�cation of workby Diego Fontaneto; credit b: modi�cation of work by U.S. EPA; scale-bar data from Cory Zanker)

The rotifers are �lter feeders that will eat dead material, algae, and other microscopic living organisms,and are therefore very important components of aquatic food webs. Rotifers obtain food that is directedtoward the mouth by the current created from the movement of the corona. The food particles enter themouth and travel to the mastax (pharynx with jaw-like structures). Food then passes by digestive andsalivary glands, and into the stomach, then onto the intestines. Digestive and excretory wastes are collectedin a cloacal bladder before being released out the anus.



:

Watch this video1 to see rotifers feeding.

Rotifers are pseudocoelomates commonly found in fresh water and some salt water environments throughoutthe world. Figure 5 shows the anatomy of a rotifer belonging to class Bdelloidea. About 2,200 species ofrotifers have been identi�ed. Rotifers are dioecious organisms (having either male or female genitalia) andexhibit sexual dimorphism (males and females have di�erent forms). Many species are parthenogenic andexhibit haplodiploidy, a method of gender determination in which a fertilized egg develops into a femaleand an unfertilized egg develops into a male. In many dioecious species, males are short-lived and smallerwith no digestive system and a single testis. Females can produce eggs that are capable of dormancy forprotection during harsh environmental conditions.

1http://openstaxcollege.org/l/rotifers



Figure 5: This illustration shows the anatomy of a bdelloid rotifer.



Glossary

De�nition 5: Annelidaphylum of vermiform animals with metamerism

De�nition 5: captaculatentacle-like projection that is present in tusks shells to catch prey

De�nition 5: clitellumspecialized band of fused segments, which aids in reproduction

De�nition 5: conispiralshell shape coiled around a horizontal axis

De�nition 5: coronawheel-like structure on the anterior portion of the rotifer that contains cilia and moves food andwater toward the mouth

De�nition 5: ctenidiumspecialized gill structure in mollusks

De�nition 5: mantle(also, pallium) specialized epidermis that encloses all visceral organs and secretes shells

De�nition 5: mastaxjawed pharynx unique to the rotifers

De�nition 5: metamerismseries of body structures that are similar internally and externally, such as segments

De�nition 5: Molluscaphylum of protostomes with soft bodies and no segmentation

De�nition 5: nacrecalcareous secretion produced by bivalves to line the inner side of shells as well as to coat intrudingparticulate matter

De�nition 5: Nemerteaphylum of dorsoventrally �attened protostomes known as ribbon worms

De�nition 5: parapodium�eshy, �at, appendage that protrudes in pairs from each segment of polychaetes

De�nition 5: pilidiumlarval form found in some nemertine species

De�nition 5: planospiralshell shape coiled around a vertical axis

De�nition 5: planuliformlarval form found in phylum Nemertea

De�nition 5: radulatongue-like organ with chitinous ornamentation

De�nition 5: rhynchocoelcavity present above the mouth that houses the proboscis

De�nition 5: schizocoelomcoelom formed by groups of cells that split from the endodermal layer

De�nition 5: seta/chaetachitinous projection from the cuticle

De�nition 5: trochophore�rst of the two larval stages in mollusks



De�nition 5: veligersecond of the two larval stages in mollusks


(1104L) Phylum Rotifera & Phylum Platyhelminthes - cnx.org · OpenStax-CNX module: m63244 1 (1104L) Phylum Rotifera & Phylum Platyhelminthes * Je rey Mahr Based on Superphylum Lophotrchozooa

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