23G 



COMPARATIVE ANATOMY. 



posterior portion of the body, and in the Ostracoda, as in many 

 Phyllopoda, the two halves of this structure are movably connected 

 with one another on the dorsal surface. In them the valves of the 

 shell extend also over the anterior portion of the body, and so enclose 

 the whole animal. 



The very peculiar modifications of the integument in the Cirri- 

 pedia are structures of this kind. The fold, which in the Ostracoda 

 is formed into a bivalve shell, is seen in an early stage in the Cirri- 

 pedia. When the animal becomes attached by its antennae, the 

 dorsal portion of the integument is converted into a wide sack or 



mantle (Fig. 119, d ef) which 

 encloses the body; the sack and 

 the enclosed body are united 

 to one another in the cephalic 

 region only. The portion of 

 this sack, which carries the 

 primitive point of attachment, 

 either remains soft and grows 

 out into a stalk - like organ 

 (Lepadidae), or is converted 

 into a broad basal surface 

 (Balanidae). In many Cirri - 

 pedes the whole mantle re- 

 mains soft (Alepas). But 

 most of them acquire firm 

 calcified shell - pieces, which 

 are developed in the outer 

 lamella of the mantle. The 

 rest of the body, with the 

 post-abdomen, which is beset with tendril-like feet, is enveloped in this 

 mantle, which has the form of a shell, and is in connection with the 

 surrounding medium by means of a cleft, which can be closed at will. 

 In the Rhizocephala this mantle-like envelope forms a tube, which 

 is smooth externally, or a disc, which is marked off into symmetrical 

 lobes. A narrow orifice, like that which leads into the mantle cavity 

 of the Cirripcdia, leads into a space which corresponds to this mantle 

 cavity, and functions as a marsupial pouch. In the Cirripedia part 

 of the typical crustacean body, with its appendages, is enveloped by 

 the mantle, and permanently retains its structure ; but in the Rhizo- 

 cephala the whole surface of the jointed body becomes converted 

 into the mantle. 



Another phaenomenon, due to the mode of parasitism, is corre- 

 lated with this atrophy of the body ; numerous tubules are developed 

 by the part of the head which is sunk into the body of the host ; 

 these tubules, which partly anastomose into retiform plexuses, extend 

 to the enteric canal of the host, and surround long tracts of it. Thus 

 an apparatus is formed which extracts nutrient fluid directly from 

 the enteron of the host, and carries it to the parasite. Many other 

 examples of the degenerating influence of the parasitic habit may be 

 observed ; as, for example, the varied forms of the Siphonostoma. 



c (I J a o 



Fig. 119. Transverse section of a Balanus. 

 a Month of the animal. b b 1 Apjtend- 

 ages converted into tendril-like structures. 

 c Head of the animal, d Mantle-like en- 

 velope. e e Movable valves which close 

 the shell. // External shells, vi Muscles 

 (after Darwin). 



