262 PHYLUM ANNELIDA 



Each is a tuft of hollow thread-like branches, through the 

 thin walls of which the red blood shines. The afferent 

 branches to the gills all come from the ventral vessel ; the 

 first six efferent vessels from the gills open into the sub- 

 intestinals ; the posterior seven open into the dorsal vessel. 

 As the papillae on the proboscis are hollow and contain 

 vessels, they are doubtless of respiratory significance. 

 Indeed, the gills may be regarded as exaggerated papillae. 



Excretory and reproductive systems. — In the anterior 

 region, in segments 4-9, there are six pairs of nephridia, 

 of which the foremost seems in process of degeneration. 

 Each consists of three parts — a funnel opening into the 

 body cavity, a glandular portion, and a bladder com- 

 municating with the exterior. 



The sexes are separate and similar. The reproductive 

 organs are very simple, and arise by proliferation of the 

 peritoneal membrane beside the blood vessels supplying 

 the funnels of the nephridia. The reproductive cells are 

 liberated into the body cavity, and there matured. They 

 pass out by the nephridia, and may be temporarily stored 

 in the bladder portions of all but the first. Little is known 

 in regard to the development, beyond the fact that the 

 young are for a time free-swimming pelagic forms. 



Development of Polychaeta. — As an example of the development 

 of the marine Cha^topods, we may take Eupomatus. Here segmenta- 

 tion is complete, but somewhat unequal, and results in the formation 

 of a blastula, with its upper hemisphere composed of small (ectodermic) 

 cells, and the lower of large (endodermic) cells. Among these latter 

 are two spherical cells — the primitive mesoderm cells, which at a much 

 later (free-swimming) stage give rise to mesoderm bands. A gastrula 

 is formed by invagination. Partial closure of the blastopore forms a 

 primitive mouth and anus to the archenteron. The anus becomes 

 closed. At a later stage the aboral region of the gastrula tilts forward, 

 an insinking of the ectoderm forms the stomodasum, and a posterior 

 ectodermic invagination opens to form the hind-gut and anus. The 

 larval gut so formed has a distinct v'entral curve. Cilia appear on 

 the surface at an early stage, and now form a distinct pre-oral ring or 

 prototroch, and also a less constant post-oral ring or metatroch. A 

 tuft near the anus forms a telotroch. Another tuft is formed at the 

 apex of the pre-oral region, where an ectodermic thickening takes 

 place ; this gives rise to an apical ganglion, with which sensory 

 structures are often associated. Very prominent are a pair of larval 

 excretory tubes — protonephridia — which open near the anus. The 

 larva so formed is a typical Trochosphere, such as occurs in the great 

 majority of Polychajta, in a more or less modified guise in many other 



