ANNELIDA 



89 



it does from a region just ventral to the proctodeum, the 

 endoderm is hollowed, if necessary, to produce the enteron, 

 the nervous band is expanded into segmental ganglia the 

 cerebral ganglia are formed from their anterior ends and the 

 coelom is divided by the formation of the septa, and at the 

 same time distended by coelomic fluid. Growth in length is 

 associated with special cells derived from the posterior end 

 of the ovum and the dorsal lip of the blastopore. These yield 

 the mesoderm and the ectoderm ; the mesoderm and the 

 nerve cord and nephridia in special band-like series of cells. 

 The bands become differen- 

 tiated in front while they are 

 being formed posteriorly. 



Nereis. It may be con- 

 venient to examine a species 

 of the marine bristle worms. 

 The genus Nereis is common 

 between tide-marks. It will 

 be noted that the Polychaet 

 differs from the Oligochaet 

 by having the lateral body 

 wall in each segment produced 

 into parapodia bearing many 

 bristles or chaetae. A closer 



inspection will show that the parapodium divides into a 

 dorsal notopodium and a ventral neuropodium, that each is 

 bilobed and bears a process called a cirrus. The mouth is 

 on the first segment, and it leads into a protrusible buccal 

 cavity and this again into a pharynx. At its anterior end 

 the pharynx is armed with two strong chitinous jaws, and the 

 buccal sac bears many small denticles. The prostomium is 

 furnished with two pairs of eyes and bears processes known 

 as palps and tentacles (fig. 43). 



General Considerations. Marine worms often have a larval 

 stage in development. The larva is a trochophore and is very 

 similar to that of the Mollusca. It consists of an ectoderm 

 and an endoderm communicating at a mouth and anus with 

 the exterior, and is furnished with one or more rings of cilia. 

 The mesoderm is primary, and the cavity is a primary body 



Apical 

 ganglion 



Mouth Proto- 



nephridium 



FIG. 45. Trochophore larva of a 

 Polychaet. Compare with fig. 36. 



