INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION SPIRALIA

PHYLUM ANNELIDA

SUBPHYLUM ACLITELLATA

CLASS MYZOSTOMIDA

A small group of echinoderm ectoparasites (associated with sea lilies, starfish, and brittle stars) characterized by a simplified body Structure resulting from a parasitic lifestyle. Their body length ranges from a few millimeters.

Anatomy. The body is flattened, disc-shaped, or rounded (Fig. 177). The anterior end, bearing the Pharynx, is retracted into a dermal pocket from which it can be everted outwards. Segmentation is not outwardly apparent, but there are four to five pairs of unbranched parapodia bearing setae; certain species also possess numerous lateral tentacles.

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Fig. 177. Structure of Myzostomum elegans (after Hartwich): a - general ventral view of the male; b - schematic diagram of anatomy; c - Nervous system:

1 - opening through which the anterior body end is protruded; 2 - lateral tentacles; 3 - parapodia; 4 - lateral Organs; 5 - anus; 6 - male genital opening; 7 - Mouth; 8 - pharynx; 9 - intestinal branches; 10 - cloaca; 11 - coelomic branches;

12 - supraesophageal ganglion; 13 - fused ventral nerve cords; 14 - nerves to the parapodia

The Skin-muscular sac exhibits a typical annelid structure. Body cavity. The coelom is unsegmented, yet forms branching outgrowths, and possesses one or several pairs of coelomducts.

Digestive System. The mouth leads into a muscular pharynx through which the parasite ingests nutrients from the host's body. The midgut features from two to 13 pairs of lateral diverticula where these nutrients are stored. The hindgut forms an expansion—the cloaca—into which the coelomducts open.

The Circulatory system is absent.

The Nervous System consists of a simple ganglion linked to two closely appressed ventral nerve cords, which are connected by 10 transverse commissures. In Representatives of the genus Myzostomum, the ventral cords are markedly shortened and have lost metamerism, transforming into a solid mass from which 10 pairs of nerves arise (Fig. 177, c).

Reproductive System. Myzostomids are gonochoric (separate-sexed); males possess Testes and a copulatory organ used to pierce the body wall of the female and inject sperm beneath her skin. The Ovaries of females (one to two pairs) are situated within the coelomic epithelium. Mature eggs enter

the coelom, where Fertilization takes place, and are subsequently released to the outside via the coelomducts and cloaca.

Development. A trochophore larva hatches from the egg; the subsequent stage—the metatrochophore—possesses five ciliated rings, and eventually develops into a juvenile worm.

Previously, myzostomids were regarded as polychaetes modified by the Transition to parasitism and were assigned the rank of a subclass within the Class Polychaeta. However, the results of molecular genetic analyses refute their affinity with polychaetes.