HIRUDINEA OR DISCOPHORA. 223 



seeking the depths as age advances. According to some, the larva 

 represents a primitive unsegmented ancestral Annelid, with medusoid 

 affinities ; according to others, the larval characteristics are adaptive to 

 the mode of life, and without historic importance. 



Protodrilits is even smaller than Polygordius, with more cilia, mobile 

 tentacles, and two fixing lobes on the posterior extremity ; the move- 

 ments are Turbellarian-like, the reproductive organs hermaphrodite, 

 the development direct. Histriodriliis is parasitic on the eggs of the 

 lobster, and its affinities are doubtful. 



Appendix (2) to Chtrtopoda. 



Parasitic and Degenerate Chastopods. 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 Chretopod 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 to the anal aperture. The testes are paired, branched, and ven- 

 tral, 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. J/. alatitin. 



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



position, and have either become extinct or converted into 

 protandric hermaphrodites, e.g. J7. cirrifentni. 



Class HIRUDINEA or DISCOPHORA. Leeches. 



This class includes forms in which the body is oval and 

 flattened, usually devoid of seta or gills, and marked ex- 



