264 



COMPARATIVE ANATOMY 



CHAP. 



B 



witli long sensory hairs ; three of these cells lie near the shell process, and the fourth 

 near the middle line. Between the two more median sensory cells a long adhesive 

 filament projects from the opening of the gland which secretes it. Behind this 

 gland are found — (1) the oral sinus ; (2) a small prominence, the pedal swelling ; 



(3) the ciliated lateral pits, 

 H one on each side ; and (4) 



furthest back of all, the 

 ciliated shield or patch. 

 Between the mantle and 

 shell the embryonic adduc- 

 tor runs across from the one 

 valve to the other. Besides 

 these are only found a few 

 isolated muscle fibres, and 

 the rudiment 'of the mid- 

 gut, the latter as an epi- 

 thelial vesicle, which be- 

 comes entirely separated 

 from the ectoderm, and in 

 no way communicates with 

 the exterior. 



The embryo at this stage 

 leaves the gills, at the same 

 time emerging from the egg 

 shell. Its adhesive filament 

 floats in the water. If a 

 passing fish comes in contact 

 with such an embryo, the 

 latter can, by closing its 

 shell, attach itself by means 

 of the triangular processes 

 mentioned above, to its in- 

 tegument, into which the 

 spines on these processes 

 penetrate. The embryo of 

 Anodonta attaches itself 

 chiefly to the fins, that of 

 Unio to the gills of the fish. 

 The epithelium of the part 

 of the fish attacked grows 

 very rapidly in such a way 

 as in a few hours to surround the parasite completely. The embryonic false mantle 

 grows out from each valve of the shell as a fungus-like body to penetrate the tissues 

 of the host, and probably serves for nourishing the embryo. During this endo- 

 parasitic life, which lasts for several weeks, the transformation of the embryo into 

 the young Mussel is completed. In the course of this process of transformation 

 some larval organs are resorbed, and also serve for nutrition ; first the sensory cells 

 disappear in this way, then the gland of the adhesive filament with the remains of 

 the filament itself, then the adductor, and finally the false mantle. The rudiment 

 of the definitive mantle and .shell then appear. The vesicular mid-gut joins the oral 

 sinus ; the pedal swelling grows into the linguiform foot, and, in this, the rudi- 

 mentary byssus gland appears as an ingrowth of the epithelium. The rudiments 

 of the inner branchial leaves, the digestive gland, the nephridium, the heart, the 



~C<|^J^'V 



Fig. 220.— GlooMdium larva of Anodonta, from the outer leaf 

 of the gill of a female. A, from below, the shell being open 

 (after ScMerholz). B, in optical transverse section (after 

 Flemming). 1, Sensory setfe ; 2, adhesive filament ; 3, shell- 

 process ; 4, false mantle ; 5, lateral pits ; 6, oral sinus ; 7, pedal 

 swelling ; 8, ciliated patch ; 9, embryonic adductor ; 10, shell. 



