2Q2 



THE ECHINOIDEA 



cords. This group of sinuses is known as the " pseudhaemal 

 system." 



The function of the haemal and pseudhaemal systems has been 

 much debated, and the relations of their various members are still 

 uncertain. The dorsal organ is sometimes said to be a kidney, 

 as by Hamann (38) and the Sarasins (72). Hartog (39) has 

 supported this by claiming that the circulation is outward 

 through the madreporite ; but Cuenot and Ludwig (62) main- 

 tain that the current is inhalent. Leipoldt and Prouho (70) 

 point to the absence of any glandular epithelium and of any 

 connection between the cavity of the organ and the general body- 

 cavity. They therefore deny that the organ is a gland, and regard 

 its function as the making of amoeboid cells for the perivisceral 

 fluid. It therefore seems most probable that the haemal system 

 distributes nutrient material through the body, both in solution 

 and by corpuscles (see also Durham, 11 of previous list, p. 36). 



A third circumoesophageal ring is that of the Nervous System 

 (Fig. VII. n.r). This is placed below the water- vascular ring. From 

 it five radial nerve cords (r.n.c), the ambulacral nerves, pass up the 

 inside of the test, between it and the ambulacral water-vascular 

 vessel. Branches from the ambulacral nerve cord pass right and left 

 to the ampullae, and give off smaller branches which pass through 



the pores to the suckers 

 (Fig. VIII.). The branches 

 fork, one half running up 

 the podion and the other 

 expanding over the surface 

 of the test as a plexus, 

 which controls the move- 

 ments of the spines and 

 the pedicellariae. A small 

 nerve ring (n.sp) surrounds 

 the base of each spine. 



The Generative Organs 

 are large and simple. They 

 consist of five branching 

 glands (Fig. VII. g), which 

 lie attached to the inter- 

 ambulacral plates in the upper part of the body-cavity. Each of 



ep... 



FIG. VIII. 



Diagram of innervation of spines in Echinus, pi, 

 ambulacral plate ; ep, epidermis ; in, muscles of 

 spines ; n, branch of nerve passing out through 

 pore ; n.pd, branch of nerve to tube foot ; n.x, branch 

 of nerve running across test to n.sp, the spinal 

 nerve ; pd, podion ; sp, base of spine ; pi, ambulacral 

 plate with (6) boss of tubercle and (m) mamelon. 



the five organs opens to the exterior by a single tube which passes 

 through the pore in a genital plate. The young of Echinus are 

 free-swimming plutei, and undergo a metamorphosis during the 

 development and resorption of the pluteal skeleton and its append- 

 ages (cf. development of Echinocyamus, Chapter VIII. pp. 15-17). 



Respiration is largely effected by the aeration of water in the 

 podia ; but in addition to this there is a series of five pairs of small 



