374 CH.ETOPODA. 



the marine Chcetopoda, the ova or spermatozoa originate on the body 

 wall (fig. 303) from cells of the peritoneal membrane, either in the 

 anterior segments alone or along the whole length of the body. The 

 generative products then become free in the body cavity, attain 

 maturity, and pass through the segmeiital organs to the exterior. 

 Only a few Chcetopoda, as Eunice and Syllis vivipara, are viviparous, 

 all the rest are oviparous ; many lay their eggs in connected groups, 

 and carry them about with them, while the Oligochceta lay theirs in 

 cocoons. 



Development. The segmentation is unequal. A primitive streak 

 is very generally developed, though sometimes not until the embryo 

 has left the egg. It arises on the ventral side in consequence of the 

 development of a middle layer and from neutral plates of the upper 



layer. 



Excepting in the Oligochceta, the young forms undergo a metamor- 

 phosis and after leaving the egg appear as ciliated larvpe, which are 

 provided with mouth and alimentary canal, and essentially resemble, 

 with some modifications, Loven's larva. 



The capability of renewing lost portions of the body, more espe- 

 cially the posterior part of the body and different appendages, seems 

 to be generally distributed. The Lumbricince and. certain marine 

 Worms (Diopatra, Lycaretus] are even able to replace the head and 

 the anterior segments, with the brain, cesophageal ring, and sense 

 apparatus. 



Fossil remains of Chcetopoda are found from the Silurian onwards 

 in the most different formations. 



Order 1. POLYCH.ETA.* 



Marine Chcetopoda, with numerous setce embedded in the parapodia, 

 usually with distinct head, tentacles, cirri, and branchiae. They are 

 for the most part dicecious, and develop with metamorphosis. 



The marine Chcetopoda must be considered as belonging to a 

 higher grade of life, on account of the sharp distinction of the head 

 which is composed of the pra?stomiuni (prseoral lobe) and oral 

 segment (in the Amphinomidce several succeeding segments are also 

 included), and of the presence of the tentacles, tentacular cirri and 



* Aiuloum et Milne Edwards, ' Classification dcs Annelides et description 

 rles cclles qui habitent les cotes de la France," Annales dcs So. Nat., Tom 

 XXVII. to XXX., 1832-83. Delle Chiaje, " Descrizioni e notomia de-'li 

 animali scnza vertebre della Sicilia citeriore," Napoli, 1841. Quatrefages, 

 " Histoire naturelle des Armeies," Tom. I. and II., 1865. Also the numerous 

 writings of E. Grube and E. Clapare.de. 



