V. Echinoidea. 1 1 



as well as the inward current. There are no valves in the water- vessels which 

 regulate the flow into the tube-feet, as in Asterids ; but the cavities of the ampullae 

 are traversed by muscular fibres, or by perforated partitions of connective tissue 

 covered with muscular fibres. The so called Polian vesicles are respiratory or- 

 gans: their cavity is lined by ciliated epithelium and contains the same amoeboid 

 cells as the water- vascular ring with which it communicates. The thickened con- 

 nective tissue layer of the upper wall contains blood lacunae derived from the oral 

 blood ring, and sends numerous projections into the water-vascular cavity beneath. 

 The so called ambulacral gills round the mouth are outward prolongations of 

 the body cavity, without any musculature in their walls, but are ciliated inside 

 and out. Both the connective tissue and the outer epithelium contain wander-cells 

 with strongly refractile granules which are probably in course of migration to the 

 exterior ; while the cavity of the gill is filled with irregular cells composed of clear 

 protoplasm like those in the enterocoel. The schizocoel spaces, chiefly 

 observed in S. granularis, are as follows : an oesophageal ring and 5 double ra- 

 dial canals, an anal ring with 5 branches to the genital organs. There is also 

 an intestinal lacuuar system , containing similar cells , but a different fluid from 

 that of the schizocoel spaces; it consists of the anal ring and the so called dor- 

 sal and ventral vessels , which arise from the oral blood ring ; the former not far 

 from its origin gives off a branch which ramifies on the surface of the axial organ. 

 Both vessels, and also the oral and anal ring, consist essentially of intercommuni- 

 cating spaces in the connective tissue, which have no endothelial lining; their 

 walls contain longitudinal muscle fibres situated beneath the coelomic epithelium. 

 The blood cells are amoeboid and consist of clear homogeneous protoplasm, occur- 

 ring both in the intestinal lacunae, and in all the schizocoel spaces. Granular ele- 

 ments (Plasma wander-cells) also occur in the connective tissue and in the lacunae. 

 Beneath the periproct is an annular schizocoel space in the wall of which is the 

 anal blood-lacunar ring. This is enlarged in the interradii , both projecting into 

 the schizocoel ring and giving off a branch which passes along the wall of the 

 genital duct and ramifies over the corresponding gland. In the madreporic inter- 

 radius it is joined by the superficial lacunae of the axial organ, which thus estab- 

 lish a communication between it and the oral ring ; and it is also in connection 

 with the lacunae on the rectum. The double radial canals of the schizocoel system 

 are situated respectively on the outer and inner faces of the nerves and , like 

 the anal ring , have a definite endothelial lining . They end blindly against each 

 radial plate where it is pierced by the water-vessel , and the inner one is oblite- 

 rated at the surface of the lantern while the outer one forms an oral ring sinus. 

 Along the rays the inner canal sends a branch to each tube-foot, between its nerve 

 and water-vessel. - - The structure of the so called ovoid gland or axial organ 

 was studied in Arbacia pustulosa and in S. granularis. Its most peripheral portion 

 consists of blood-lacunae which communicate above and below with the anal and 

 oral blood-rings respectively. The greater part of the organ is formed by a mesh- 

 work of connective tissue fibres, with some larger cavities in the centre which are 

 lined by an endothelium of connective tissue cells. The smaller spaces of the 

 meshwork contain the same clear amoeboid cells as the water-vessels, blood- 

 spaces, and body cavity, and also the plasma wander-cells with strongly refractile 

 granules. In S. echinus the former are the most abundant, and in A. the latter. 

 Pigment is most abundant towards the periphery of the organ, and its presence in 

 quantity seems to be accompanied by degeneration of the cells containing it. The 

 organ itself is probably excretory in nature, waste products being carried through 

 it out of the blood by the amoeboid and wander-cells. - - The author's description 

 of the structure of the walls of the digestive tube agrees very well with that 



