HIRUDINEA OR DISCOPHORA. 235 



Appendix (2) to Chcetopoda 



Parasitic and Degenerate Chaetopods. MYZOSTOMATA 



The remarkable forms (Myzostoma] included in this small class, live 

 parasitically on feather-stars, on which they form galls. They are 

 regarded as divergent offshoots from primitive Annelids, the larval form 

 showing some distinctly Chsetopod characters. The minute disc-like 

 body is unsegmented, and bears five pairs of parapodia, each with a 

 grappling hook, with which four pairs of suckers usually alternate. 

 There are also abundant cirri. The skin is thick, the body muscular, 

 the nervous system is concentrated in a ganglionic mass, which encircles 

 the gullet, and gives off abundant branches. There is a protrusible 

 proboscis and a branched gut ; the mouth and anus are ventral. The 

 ova arise in the reduced body cavity, and pass by three meandering 

 oviducts ^ the anal aperture. The testes are paired, branched, and 

 ventral, with associated ducts, which open anteriorly on the side of 

 the body. 



The series are united, but there is marked protandry. The very 

 young forms, originally described as "dwarf males," contain sperma- 

 tozoa, and are often carried on the back of the mother ; as they grow 

 older they become hermaphrodite, and later the power of forming 

 spermatozoa is lost and the animals become female. 



It must be allowed, however, that all would not agree with the above 

 summary. Thus Beard says : " The various kinds of parasitism 

 presented by the numerous species of Myzostoma, have led in some 

 cases to the preservation of the males, in others to their extinction, in 

 yet others to their conversion into hermaphrodites." He distinguishes 



1. Purely dioecious forms with small males, e.g. M. pulvinar. 



2. Hermaphrodite forms and true males, which remain males, e.g. 



M. glabrum. 



3. Hermaphrodite forms and males, which, retaining their positions 



on the hermaphrodites, afterwards become female, e.g. M. alatum. 



4. Hermaphrodite forms, in which the males have lost their dorsal 



position, and have either become extinct or converted into 

 protandric hermaphrodites, e.g. M. cirriferum. 



Class HIRUDINEA or DISCOPHORA. Leeches 



This class includes forms in which the body is oval and 

 flattened^ usually devoid of setce or gills, and marked ex- 

 ternally by rings which are much more numerous than the 

 true segments. The body cavity is much reduced and broken 

 up (except in Acanthobdella), and may communicate indirectly 

 with the well-developed vascular system. The nephridia are 

 numerous and segmentally arranged. There are usually two 

 suckers, one at each end of the body, the anterior being formed 



