206 SEGMENTED WORMS OR ANNELIDA. 



red blood shines. As the papillae on the proboscis are 

 hollow and contain vessels, they are doubtless of respira- 

 tory significance. Indeed, the gills may be regarded as 

 exaggerated papillae. 



Excretory System. 



In the anterior region, from the fifth to the tenth seg- 

 ments, there are six pairs of nephridia. Each consists 

 of three parts a funnel opening into the body cavity, a 

 glandular portion, and a bladder communicating with the 

 exterior. 



Reproductive System. 



The sexes are separate and similar. The reproductive 

 organs are very simple, and lie on the peritoneal mem- 

 brane of the body cavity. They are developed in close 

 association with the nephridia. The reproductive cells are 

 liberated into the body cavity, and there matured. In 

 August and September they pass out by the nephridia. 

 Little is known in regard to the development, beyond 

 the fact that the young are for a time free swimming 

 pelagic forms. 



Development of Polychceta. 



As an example of the development of the marine Chsetopods, we may 

 take Eupomatus, which has been investigated by Hatschek. Here 

 segmentation 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 mesoblasts. Invagina- 

 tion takes place in the usual way to form a gastrula, the primitive meso- 

 blasts divide and form mesoblastic bands. During these processes the 

 external form has altered considerably. The apical (aboral) region of 

 the gastrula becomes tilted forward, an ectodermic invagination arises 

 posteriorly, and uniting with the archenteron produces hind-gut and 

 anus, while a similar insinking anteriorly, in the region of the blasto- 

 pore, forms fore-gut and mouth. The larval gut so formed has a 

 distinct ventral curve. Cilia appear on the surface at an early stage, and 

 now form a distinct pre-oral ring, and also a less constant post-oral ring. 

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

 this gives rise to an apical ganglion with which sensory structures are 

 often associated. The mesodermic bands give rise to muscle cells used 

 in swimming, and also to the " head kidneys " a pair of larval ex- 

 cretory tubes. The larva so formed is a typical Trochosphere, such as 

 occurs in the great majority of Polychseta, in a more or less modified 



