362 ANNELIDA. 



Development. Segmentation is irregular and complete, and results 

 in the formation of an embryo, which is enclosed in three egg-mem- 

 branes. The embryo has a small, somewhat long body, armed with 

 .small spines at the anterior pole, and containing a central granular 

 mass (embryonic nucleus) (fig. 292). It passes into the intestine of Am- 

 phipods (Ech. proteus, polymorphus), or of Isopods (Eck. angustatus), 

 and there becomes free, bores through the wall of the intestine, and 

 after losing the embryonic spines, develops to a small elongated larva, 

 which, like a pupa, lies in the body cavity of the small Crustacean 

 with its proboscis retracted and surrounded by its firm external 

 skin as by a cyst (fig. 293). The skin of the larva gives rise only 

 to the integument, the vessels and the lemnisci of the adult ; while 

 all the other organs enclosed within the dermal muscular envelope, 

 viz., the nervous system, the sheath of the proboscis, and the gene- 

 rative organs, are developed from the so-called embryonic nucleus. 

 It is only after their introduction into the intestine of fishes (Ech. 

 proteus) or of aquatic birds (Ech. polymorphus), which feed on these 

 Crustacea, that the larvae attain to sexual maturity, copulate, and 

 reach their full size. 



The numerous species of the genus Erhhborhyiirii* 0. F. Miiller live prin- 

 cipally in the alimentary canal of different Vertebrate ; the gut wall may be 

 as it were sown with these animals. Erli. polij \nt\rpliux Brems., in the intestine 

 of the duck and other birds, also in the crayfish. Ecli. proteux Westrumb., 

 Ecli. amjiixttitiix Rud., in fresh-water fish. Ecli. ffi-t/<i.i Goeze, as large as an 

 Ascarix Iwnibricoides, in the small intestine of the pig. According to 

 A. Schneider, the embryo completes its development in the maggot. 

 Lambl found a small sexually immature Echinorhyiichus in the small intestine 

 of a child which died of leukaemia. 



CLASS III. ANNELIDA. 



Segmented Vermes with brain, circum-ce.sophageal ring, ventral nerve 

 cord, and vascular system. 



The larva of Loven and its development seems to throw light 

 upon the organization of the Annelida and their relations to the 

 lower worms and to the Rotifera ; and further makes evident the 

 relationship of the Annelida to the Gephyrea, a group of worms 

 which possess an elongated body devoid alike of external and 

 internal segmentation, and, as an equivalent of the ganglionic chain, 

 a ventral nerve trunk, which is usually \iniformly covered with 

 ganglion cells. 



The body of Loven's larva, from which we must derive the body 

 of Annelids, is unsegmented, and represents mainly the Annelid head. 



