ANNELIDA. 



215 



they are drawn inwards, effectually protect the neighbouring soft 

 parts from laceration. 



(259.) In the Aphrodite above alluded to we have an additional 

 appendage developed from the upper part of each lateral oar, in the 

 shape of a broad membranous scale, which, arching inwards over 



Fig. 89. 



Fig. 90. 



the back {Jig. 89, c), forms with 

 its fellows a series of imbricated 

 plates, or Elytra, as they are tech- 

 nically named (Jig. 88, A) ; and 

 beneath these the branchial organs 

 are lodged. Each of the elytral 

 scales is formed by a double mem- 

 brane, between the laminae of which 

 at certain seasons the eggs are 

 found to be deposited ; a situation 

 evidently adapted to ensure the ex- 

 posure of the ova to the influence of the surrounding element, 

 and thus to provide for the respiration of the embryo.* 



(260.) The structure of the mouth in the Dorsibranchiate Anne- 

 lidans is very peculiar. The 

 first portion of the alimentary 

 canal or stomach, as it is most 

 erroneously called by some 

 writers, is muscular ; and 

 certainly, when seen in a dead 

 Annelide, it might easily be 

 taken for a digestive cavity. 

 Nevertheless, during life, this 

 part of the alimentary ap- 

 paratus is destined to a widely 

 different office ; for it is so 

 constructed, that at the will 

 of the animal it can be com- 

 pletely everted, turned inside 

 out, and, when thus pro- 

 truded externally, it forms a 

 very singular proboscis, used 

 in seizing food, and frequent- 

 ly armed with powerful teeth of singular construction. The an- 

 nexed figure (Jig. 90, A), representing the head of one of these 



* Milne Edwards, Ann. des Sciences Nat. vol. xxvii. 



