//. ACEPIIALA: PRGTOCHOAX'HI^. 



365 



the shell ffig. 354j. After escape from the gills they swim about by 

 opening and closing ilie shells, and by 

 means of the hooks attach tliemselves 

 to passing fish. They i)TOduce an ulcer 

 in the skin of the fish in which they^jf 

 grow, and by renewal of the shell and 

 the adductor muscles attain the de- 

 finitive condition. After this metamor- 

 phosis they fall to the bottom, to live 

 henceforth half buried in the mud. 



Structure of gills, hinge, edge of ^^^ ^^_^ 

 mantle, and adductor muscles have been 

 used lis basis of classification, the usual 

 divisions being founded on characters derived from only one of these 

 organs. 



Order I. Protochonchiae. 



The primitive cliaracter of tliese forms is shown b}' the struc- 

 ture of the gills, which are either ctenidia (Protobranchiata) or 



ad 



- ^lochidium of Anodnnta. 



(From Balfour.) ad, adductor ; by, 

 byssus ; s, sense hairs ; sh, shell. 



Fig. 355. — Anatomy of Nucula. f Aftor Drew.) aa, anterior adductor; h, byssal gland; 

 c, cerebral ganglion; at, cteriidiuni : f, foot; li, heart: /, labial palpus; o, fitocyst; 

 p, pedal ganglion; pa, posterior adductor; s, stomach ; t, appendage of palpus; 

 V, visceral ganglion. 



filamentary (Filibranchiata), yet here and there, as in the scal- 

 lop.? and oysters (Pseudolamellibranchiata), the fusion of gill ii];i- 

 inents is already begun. Hinge and ligament are syinmctrical 

 with regard to the umbo, or vary little from symmetry. The hinge 

 may be lacking, and the ligament is wholly or in jiart interi);il. 

 The mantle edges are free, and rarely is there the first trace of fusion. 



