AND THE LOWER ANIMALS. 109 



The young anodon has none of these organs ; but 

 to compensate for their absence, it possesses a very 

 peculiar apparatus for closing its shell and preventing 

 the entrance of parasitic infusoria. Each valve, which 

 is then of a triangular form, has attached to its upper 

 portion a long flexible structure, and this latter is 

 covered with strong teeth, which are arranged in 

 squares of five each — four forming the sides, and one 

 in the centre. By means of muscles for the purpose, 

 these two structures can be drawn inwards, somewhat 

 in the way in which a knife turns in its handle ; and 

 the teeth being thus firmly clenched, the shell is as 

 securely closed as if it were done by machinery. 



The larvae of oysters, teredos, and anodons are 

 retained, either in the gills or mouth of the parent, 

 from the time they leave the egg till the moment of 

 their metamorphosis ; they then lose their temporary 

 organs and assume the adult features, being sometimes 

 elevated a few degrees in the animal scale, at others 

 descending to a lower grade than they occupied as 

 larvas. The former takes place in the case of the 

 anodons, which acquire a foot, and can at all events 

 crawl along the mud ; the latter among oysters ; and, 

 still more so, among teredos. The latter, which were 

 the most highly organized of the three groups when 

 in the larval state, are by far the most degraded in 

 the adult condition. 



Thus we see, that even among Acephala, the two 

 forms of metamorphoses are presented, and that in 

 some instances the development elevates the being, as 

 in the case of insects ; in others, it is as recurrent, as 

 in the Cirrhipedia and Lerneaa. 



