THE CIRRIPEDIA 137 



integument. Its presence causes a degeneration of the muscular 

 and hypodermic tissues between it and the external cuticle, and 

 at the next moult of the crab a hole is left through which the body 

 of the parasite, protruding from the perisomatic sac, emerges on 

 the surface (Fig. 84, B). The mantle-opening is at first closed by 

 a plug of chitin, but in other respects the young Sacculina e.'-tcnni 

 has already the essential structure of the adult. 



The development of Peltogaster seems to follow much the same 

 course as that of Sacculina, except that no perisomatic cavity is 

 formed. In this genus also the parasite penetrates the cuticle and 

 reaches the exterior without any moult of the host. Very little is 

 known of the development in other genera of Ithizocephala. In 

 the genus Thompsonia the larvae are stated to reach the < '///<//>- 

 stage while still within the mantle-cavity of the parent. 



APPENDIX TO KHIZOCEPHALA. 



The genus Sphaerothylacus, described by SI niter, is parasitic on 

 a simple Ascidian (Polycarpd), living attached by ramifying roots to 

 the inner wall of the branchial sac. The globular body is enclosed 

 in a mantle which has a small opening. There are no appendages, 

 but there is a complete alimentary canal with mouth and anus, the 

 latter near the mantle-opening. The two ovaries each consist of 

 two long caeca which unite to open, with those of the opposite side, 

 into a common atrium near the mouth, surrounded by a mass of 

 glandular tissue, no doubt representing the colleteric glands. The 

 testes are paired simple tubes, opening close to the anus. The 

 nauplius larvae differ from those of the true Rhizocephala in the 

 absence of fronto-lateral horns. 



The genus Sarcotaces, Olsson, comprises two species which live 

 embedded in the muscles of fish. Their structure is very imper- 

 fectly known, but an alimentary canal is said to be present and 

 there are no " roots." The nauplius larva is without fronto-lateral 

 horns. 



The systematic position of both genera is quite obscure, and 

 only further investigation can determine what relation, if any, they 

 bear to the true Rhizocephala. 



HKMARKS ON HABITS, ETC., OF CIRPJPKDIA. 



The Cirripedia are exclusively marine, only a very few 

 species penetrating into brackish water. In correlation with 

 their sedentary habits the non - parasitic Cirripedia have 

 developed a peculiar method of feeding by sweeping the water 

 for floating particles of nutriment, with a net formed by 

 the tendril -like branches of the thoracic limbs. Most species 



