ANNELIDA 229 



posterior suckers and a hermaphrodite reproductive system, closely 

 paralleled in a subdivision of the Chaetopoda, the Oligochaeta, show 

 the specialization of the group. The Gephyrea is a group of burrowing 

 marine worms in which segmentation has been almost entirely sup- 

 pressed in the adult but is sometimes shown in the larvae by meso- 

 blastic somites and ganglion rudiments. Chaetae are lost except in 

 a few forms, but a large perivisceral coelom is preserved. 



Class CHAETOPODA 



Well-segmented Annelida, with chaetae and a spacious perivisceral 

 coelom, usually divided by intersegmental septa. 



In a typical chaetopod there is a distinct preoral region or pro- 

 stomtum and a postoral body composed of many segments. Each 

 segment owes its distinctness to the development in the larva of a 

 pair of mesoblastic somites which join round the gut, the cavities 

 which develop in them becoming the perivisceral cavity of the adult 

 segment. At the same time the larval ectoderm (epiblast) develops 

 segmentally repeated organs : the ganglia, swellings in the continuous 

 ventral nerve cords, the nephridia or excretory organs and the chaetae. 

 In the Polychaeta, one of two orders into which the Chaetopoda are 

 divided, the chaetae are borne in groups upon processes known as 

 parapodia, whose projection from the body wall is due to the de- 

 velopment of special muscles for moving the chaetae. In the other 

 order, the Oligochaeta, there are no parapodia. 



The coelom is bounded by an epithelial layer, t\it peritoneum, which 

 gives rise to the gonads, which in polychaets are usually developed in 

 most of the segments, to the yellow cells, which play a part in the work 

 of nitrogenous excretion, and to the coelomoducts by which the eggs 

 and sperm pass from the coelom to the exterior. In most of the poly- 

 chaets the eggs are fertilized externally, forming a trochosphere larva, 

 the method of reproduction thus conforming to that of other marine 

 groups. In the terrestrial and freshwater oligochaets (as in leeches) 

 fertilization is internal and the young hatch in a form resembling the 

 parent. There is no doubt that the former mode of development is 

 more primitive. 



The nephridia are essentially tubes developed from the ectoderm 

 which push their way inwards so that they project into the body 

 cavity. In some polychaets they end blindly — this is the primitive 

 condition. In the majority of chaetopods they have acquired an 

 opening (nephrostome) into the body cavity itself. It is probable that 

 in many cases the lips of the nephrostome are formed by one or more 

 mesodermal cells so that a compound tube consisting mainly of 

 ectoderm but partly of mesoderm exists (nephromixium). Nephro- 



