THE LAMELLIBRANCHIA 



called a " glochidium " (Fig. 228). This larva is characterised by 

 the possession of hooks in the middle of the lateral borders of its 

 valves and by its larval byssus which is not homologous with that 

 of other Lamellibranchs. This byssus appears to issue from, but 

 in reality winds round, the adductor muscle, and originates from a 

 single glandular epithelial cell, deeply embedded in the tissues on 



-/ s/i 



a.ad 



jj.aa 



a i 



FIG. -228. 



Two stages in the development of .lnuilunta ; both figures represent the " glochidium " stage. 

 A, when tree swimming, shows the two dentigerous valves widely open. B, a later stage, after 

 fixation to the tin of a lisli. n.ml and ail, anterior adductor muscle ; al, alimentary canal ; au.v, 

 otocyst ; lir, branchial filaments ; by, byssus ; /, foot ; nit, mantle-flap ; p.ail, posterior adductor ; 

 , teeth of the shell ; sh, shell. (From Lankester, after Balfour.) 



the dorsal and anterior side of the muscle. The glochidia swim 

 actively by clapping together the valves of the shell, and eventually 

 attach themselves to the gills or fins of a fish, and become encysted 

 in consequence of a pathological development of the epithelium of 

 their host. This parasitic existence lasts for a period varying from 

 two to six weeks, during which the glochidia are nourished by the 

 epidermic elements of their host, absorbing them by means of the 

 ectodermic cells of the embryonic mantle. During this time most of 

 the definite organs of the adult, the 

 foot, otocysts, gills, etc., which were 

 not required in larval life, are de- 

 veloped, largely as the result of the 

 proliferation of the cells of two sym- 

 metrical cavities situated behind the 

 adductor muscle. In a general way 

 the development of the organs follows 

 the normal course, but some the 

 borders of the mantle, for instance 

 are formed anew. The glochidium 



rii- rr i . , ,1 i Parasitic larva of Anodunta on the 



Shell IS not Cast Oft but persists, though eighth day of parasit ic lite ; ventral view. 



it nndprp-OPS i ronm'dprahlp fhino-p /, foot ;sr, gill-filaments; pa, new mantle ; 



lel & e ari e of, otocyst. (After Schierholtz.) 



of shape. The posterior ciliated shield 



and the byssus disappear. During the early part of the parasitic 

 life the mouth acquires an opening into the previously closed 

 endodermic cavity or archenteron, but the anus is not formed till 



