644 



ZOOLOGY 



SECT. 



beset with spines (Fig. 535, sh). At the same time the body of the 

 larva, which has hitherto been an undivided mass projecting 

 between the two valves of the shell, becomes cleft from below 

 upwards, and thus divided into a single dorsally-placed body 

 proper, and paired right and left mantle-lobes. Upon the latter 

 peculiar brush-like sense-organs make their appearance, and on 

 the ventral surface of the body is formed a glandular pouch, which 

 secretes a long thread, the provisional byssus (/). The mesoderm 

 cells give rise to a single immense adductor muscle (sm\ the fibres 

 of which extend from valve to valve. 



The larva is now called a glocMcUum: it remains in the brood- 

 pouch, nourished by a secretion from the walls of the latter, and 

 entangled with its fellows by means of the byssus At this stage 

 the outer gill appears to be stuffed full of closely aggregated sand- 

 grains. Before long the larvae are ejected through the exhalant 



B. 



^ .--sh. 



FIG. 535. A, advanced embryo of Anodonta. B, free glochidium ; ./, provisional byssus ; 

 s, shell ; sh, hooks ; sm, adductor muscle ; so, sense-organs ; w, cilia. (From Korschelt and 

 Heider's Embryology.) 



siphon, and lie in masses on the bottom until they happen 

 to come in contact with a passing Stickleback or other fresh-water 

 Fish, when they fix themselves on some part of its body by means 

 of the hooked valves. The glochidia of Unio usually become 

 attached to the gills, those of Anodonta to the skin or the fins. 

 In this position they become encysted by an overgrowth of the 

 skin or mucous membrane of the host, and are nourished by its 

 juices absorbed through processes of the mantle. They thus lead 

 a truly ectoparasitic existence for about ten weeks. 



While in this condition a metamorphosis takes place. The pro- 

 visional byssus and sense-organs disappear (Fig. 536), and imme- 

 diately posterior to the former an invagination, the stomodceum (m), 

 is formed, and soon communicates with the archenteron. The 

 posterior end of this cavity is in close contact with the ectoderm, 

 so that the anus is formed by a simple process of rupture, and 

 without the development of a proctodseum. The foot (fit) arises as 

 a median ventral elevation behind the mouth, and on each side of 



