THE ANATOMY OF SOALIBREGMA INFLATUiM. 263 



which are nearly equal in size. The branches of each gill 

 spring from a single stem, which is short and stout, and 

 soon divides into two main branches, one of which is directed 

 dorsally and the other ventrally (fig. 4). Each of these 

 usually divides again into two, and these branch freely, 

 sometimes dichotomously, or often dividing into three. 



In the living animal the gills are red and the fine branches 

 reddish yellow, due to the contained blood (Sars). The 

 gills are hollow, each containing a prolongation of the 

 coelom. Their walls are composed of single layers of epi- 

 thelial cells, within which is a delicate coelomic epithelium 

 surrounding the axial cavity. Between these two layers is a 

 thin sheet of muscle-fibres, upon the presence of which the 

 contractility of the gill depends. 



The gills are supplied with blood by four pairs of afferent 

 vessels given off from the dorsal vessel, and they return 

 blood by a corresponding number of efferent trunks to the 

 ventral vessel (fig. 14). The position of the vessels and the 

 circulation of the blood in the gills is difficult to make out 

 from my material, as the gills are almost bloodless in all the 

 specimens. 



14. Centeal Nervous System. 



Danielssen (p. 72) has given a brief account of the nervous 

 system. He figures (pi. i, fig. 3) the nerve-cord as a double 

 chain, upon which there are ganglia in the middle of each 

 segment, each giving off a pair of nerves to the body-wall. 

 From the oesophageal connectives three fine nerves are given 

 off on each side. The brain, which consists of two masses 

 connected by a transverse commissure, also gives off three 

 nerves on each side, which run forwards. 



I cannot agree with Danielssen on several of these points, 

 and especially on the ganglionation of the nerve-cord. I find 

 that the cord is of almost uniform thickness, there being no 

 ganglia visible either in dissections or in horizontal sections. 



The central nervous system closely resembles that of 



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