ADAPTATION IN CLEAVAGE. 45 



of a long tubular gland, the thread-gland, coiled several times 

 round in the right valve of the shell. The secretion of this 

 gland takes the form of a long thread, which can, apparently, 

 be respun when lost, and which floats freely in the water. There 

 are four tufts of sense-hairs on each side of the larval mantle, 

 three arranged in the form of a right-angled triangle just within 

 the outer angle of each valve, and one on either side of the 

 middle line, a trifle behind the opening of the thread-gland. 



Now all of the structures mentioned thus far, the shell, the 

 larval mantle, the adductor muscle, the thread-gland, and the 



H 



s. A 



FIG. i . Glochidium of Anodonta, one of the UnwnideB, lying on its dorsal surface, with the 

 shells expanded so as to show the soft parts. A, anterior; P, posterior; R, right; L, 

 left ; S, shell ; H, hook ; I. m., larval mantle ;/.., larval adductor muscle ; t.g., thread- 

 gland ; t., thread; s.h., sensory hairs ; m., mouth; E. A ., embryonic area. 



tufts of sense-hairs, are purely larval organs, destined to form 

 no part of the adult, but to degenerate. They are already old 

 when the life of the individual has hardly begun, and they form 

 by far the greater bulk of the larval body. The embryonic 

 material, from which the body of the adult is to develop, is all 

 included within a small area (E. A.} just in front of the poste- 

 rior angle of the valves of the shell. This area is only about 

 one-third of the length of the glochidium, and less than a third 

 of its width, when the valves are expanded ; yet within it we 

 find the rudiments of the intestine with minute liver-diverticula 

 and end-gut well marked but no opening to the exterior, of 

 the mesoblastic structures, and of the foot, mantle, and gills. 



