//. ACEPHALA: PRGTOCHONCHI^]. 



305 



the shell (fig. 354). After escape from the gills they swim about by 

 opening and closing the shells, and by 

 means of the hooks attach themseJves 

 to passing fish. They produce an ulcer 

 in the skin of the fish in which they 

 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 as basis of classification, the usual 

 divisions being founded on characters derived from only one of these 

 organs. 



Order I. Protochonchiae. 



The primitive character of these forms is shown by the struc- 

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



FIG. 354. Glochidium of Anodonta. 

 (From Balfour.) ait, adductor ; by, 

 byssus ; s, sense hairs ; sli, shell. 



FIG. 355. Anatomy of Nucula. (After Drew.) oa, anterior adductor; ft, byssal gland; 

 c, cerebral ganglion; ct, ctenidium; /, foot; 7?, heart; 7, labial palpus; o, otocyst; 

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

 r, visceral ganglion. 



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

 lops and oysters (Pseudolamellibranchiata), the fusion of gill fila- 

 ments is already begun. Hinge and ligament are symmetrical 

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

 may be lacking, and the ligament is wholly or in part internal. 

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



