CIREIPEDIA. 433 



The effect of the presence of the parasite on the host is referred to 

 on p. 445. 



It remains to notice the fact that when the Sacculina has become ex- 

 ternal, but before the plate of chitin has disappeared from the cloacal 

 opening, numbers of Cypris larvae are found to attach themselves by their 

 antennae in the angle between the mantle and the projecting edge of 

 the plate covering the cloacal opening. These have not been seen alive 

 and nothing is known of their internal structure beyond the fact that they 

 do not shed their swimming appendages or develop the pointed process of 

 the cuticle formed by larvae which attach themselves as parasites. It is 

 conjectured that these larvae are males, which in some way, at present 

 unknown, fertilize the first batches of ova. 



The name Rhizocephala and the term " mantle " as above used imply 

 definite homologies with the parts of other Cirripedes, and the question 

 arises, How far is the use of these terms justified ? Notwithstanding 

 the complexity of the metamorphosis undergone by these remarkable 

 Cirripedes there appears to be nothing in the life-history to render unten - 

 able the view that there exist in the fully formed Sacculina parts cor- 

 responding to the mantle and the head region of other forms. 



With regard to the mantle, its relation to the visceral mass bears no 

 doubt a certain resemblance to the relation of the mantle fold of other 

 Cirripedes to the contained body ; but the account which Delage gives of 

 the origin of the layers lining the brood-chamber, by delamination from 

 an outer epithelial layer, lends no support to this homology. The ganglion, 

 again, is formed as an ingrowth from this same outer layer, not in the plane 

 of the mesentery but on one side. 



It must be confessed that the homology of the " mantle " of the 

 Rhizocephala with that of other Cirripedes is uncertain ; and if this is 

 uncertain there is no satisfactory reason for regarding the pedicle and the 

 region from which the roots spring as anterior. 



The name Rhizocephala therefore, though retained here as that by which 

 the group is usually known, implies a view of the homologies of the adult 

 structure which is at least insecure. 



The Rhizocephala, parasites of other Crustacea, are themselves liable 

 to be infested by members of the Epicarida, a parasitic group of the 

 Isopoda. 



Fam. Rhizocephalidae. Degenerate Cirripedes, parasitic on Crustacea, 

 and undergoing a remarkable metamorphosis. Peltogaster Rathke, 

 irregularly cylindrical, uncompressed ; cloacal opening anterior in relation 

 to host ; on Decapoda Anomala. Parthenopea Kossmann, roughly 

 spherical, mantle opening lateral ; on Callianassa and Gebia. Sacculina 

 Thompson, much compressed laterally, cloacal opening posterior, on 

 Decapoda Brachyura. Heterosaccus O. Smith, like Sacculina, but with 

 the mantle opening widely gaping ; on Decapoda Brachyura. Lernaeo- 

 discus Miiller, mantle expanded laterally into lappets, opening posterior 

 and median ; on Decapoda Anomura. Triangulus G. Smith, resembles 

 Lernaeodiscus in many respects, but mantle opening asymmetrically 

 situated ; on Decapoda Anomura. Sylon Kroyer, egg-shaped, mantle 

 opening paired, anterior ; on Decapoda Macrura. Clistosaccus Lilljeborg, 

 irregularly oblong, mantle opening absent ; on Decapoda Anomala. 



Incertae sedis. Duplorbis G. Smith, on the Isopod Calathura Apeltes 

 Lilljeborg ; Thompsonia Kossmann ; Thylacoplethus Coutiere. 



z in F F 



