292 THE ECHINOIDEA 



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

 system." 



The function of the haemal ard 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 

 n.j>d. 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 

 Fl - viii. the base of each spine. 



Diagram of innervation of spines in Echinus. pl t TViA ftpnArutivP Orwana 



ambulacral plate ; ep, epidermis ; m, muscles of Dative Urgans 



spines ; n, branch of nerve passing out through are large and simple They 

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



of nerve running across test to n.sp. the spinal COttSlSt Ol nve branching 



nerve ; pd, podion ; sp, base of spine ; pi, ambulacral ^lor^e, /!?; 17TT /,\ ,,,^;,a, 



plate with <^boss of tubercle and (m)mamelon. glands (* Ig. Vll. g), Which 



lie attached to the inter- 



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

 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. 15-1 7). 



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 



