AIRUDINEA OR DISCOPHORA. 235 
Appendix (2) to Chetopoda 
Parasitic and Degenerate Chetopods. Myzostomata 
The remarkable forms (/yzostoma) 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 Chaetopod 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 « 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 J/yzostoma, 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 dicecious forms with small males, ¢.g. JZ. pulvinar. 
2. Hermaphrodite forms and true males, which remain males, ¢.g. 
M. glabrum. 
3. Hermaphrodite forms and males, which, retaining their positions 
on the hermaphrodites, afterwards become female, e.g. JZ. 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. AZ. cérriferum. 
Class Hirupingea or DiscopHoRA. Leeches 
This class includes forms in which the body is oval and 
flattened, usually devoid of sete or gills, and marked ex- 
ternally by vings which are much more numerous than the 
true segments. The body cavity ts 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 
