THE LAMELLIBRANCHIA 



251 



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 



-JJ.Ctd 



dr- 



FIG. 228. 



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

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

 fixation to the fin of a fish, a.atl and ad, anterior adductor muscle ; al, alimentary canal ; au.v, 

 otocyst ; br, branchial filaments ; by, byssus ; /, foot ; mt, mantle-flap ; p.ad, 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 



i 11 . ty.-, . ,-, -, Parasitic larva of Anodonta on the 



Shell IS not Cast On OUt persists, tnOUgn eighth day of parasitic life ; ventral view. 



ot 



FIG. 229. 



if nnrlpro-npc Q pmicirlAraKIp n\M*r\crei /I foot ; 0, gill-filaments ; pa, new mantle ; 



it undergoes a considerable cnange 0<>ot0 cyst. (After scinerhoitz.) 

 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 



