352 



INVEETEBEATA 



CHAP. 



endodermic rudiment is invaginated and the blastopore closes, but 

 the stoniodaeum remains as a thickened plate of ectoderm during 

 the Glochidium stage. Eudiments of three pairs of ganglia are 

 sometimes present (Anodon), and sometimes not (Unio). On each 

 side the mantle-lobe bears three sense cells with long sense hairs. 

 There is a powerful adductor muscle connecting the valves of the 

 shell. 



In the post-larval life the most marked feature is the modifica- 

 tion of the cells forming the larval mantle. These cells develop into 

 huge vacuolated columnar structures which actually absorb and 

 digest fragments of the blood cells and other tissues of the host 



Fig. 283. — Glochidium — larva of Margaritana with widely opened valves, viewed as a 

 transparent object from the dorsal surface. (After Harms.) 



adtZ, adductor muscle ; hy.f, byssus thread ; cil, ciliated patch on the ventral surface ; sA, larval 

 shell ; sL stomach ; t. teeth of shell. 



which enter the mantle-cavity of the parasite. The cells destined to 

 form the adult mantle arise from the apices of the two mantle-grooves 

 and spread downwards, displacing the larval mantle cells, which are 

 eventually shed (Pig. 284). We can only compare these rudiments 

 of the adult mantle to the imaginal discs of insects. The single larval 

 adductor muscle disappears and is replaced by the adult adductor, 

 of which sometimes the anterior and sometimes the posterior is the 

 first to be formed. The foot, gills, and otocysts arise exactly as in 

 Dreissensia. The coelom arises as two little packets of cells, one 

 on each side, closely attached to the ectoderm, from which Harms 

 supposes them to have been derived ; but this view, for reasons given 

 above, we cannot accept. The later development of these rudiments 

 is exactly the same as in Dreissensia ; the main mass on each side 



