viri EGHINODERMATA— NERVOUS SYSTEM 453 



intestinal wall (with the exception of a larger or smaller portion of 

 the hind-gut) ; from this plexus the blood collects into two longitudinal 

 vessels, one outer and dorsal and the other inner and ventral. These 

 longitudinal vessels do not lie in but on the wall of the intestine, in 

 its mesenteries. The ventral or inner vessel, along the part where 

 the accessory intestine is developed, passes on to this latter. Both 

 vessels oi^en into a blood vascular ring which encircles the oesophagus, 

 in close contact with the water vascular ring. The position of these 

 two rings and their relation to one another has already been sufficiently 

 described (p. -(•24). From the blood vascular ring five radial vessels 

 run to the radii, within the bands of connective tissue which separate 

 the pseudoha?mal canals from the radial trunks of the water vascular 

 system (Fig. 353, p. 410). In Echinoids provided with a masticatory 

 framework, the proximal portions of these radial blood vessels descend 

 in the axis of the lantern along the edges of the five single pyramids 

 forming the lantern, which are turned towards the esophagus and to 

 the nerve ring (Fig. 3.58, p. 419), before passing through the auricles 

 to be produced as mentioned into the radii. Tliey are said not to be 

 in open communication with the circular vessel, but to be divided from 

 it by a septum. The radial blood vessels during their course give oft' 

 lateral branches which run to the bases of the tube-feet. 



A lacunar plexus is also developed in the axial organ, immediatel}- 

 below the surface ; this is either in direct communication with the 

 blood vascular ring which encircles the oesophagus, or else draws its 

 blood from the dorsal intestinal vessel. The vacuolar plexus of the 

 axial sinus is further continued into the wall of the apical circular sinus 

 of the body cavity, and thence on to the wall of the genital glands. 



3 and 4. Asteroidea and Ophiuroidea. — It is doubtful Avhether a 

 blood vascular system occurs in these two classes. This system is 

 certainly wanting in the intestinal Avail. The vessels which have 

 hitherto been regarded as blood vascular, ring, and radial blood vessels 

 (running between the nerve trunks and the pseudohamal vessels) agree 

 in structure with the axial organ, and are said to be direct continua- 

 tions of this organ. As such they fall under another heading. 



5. Crinoidea. — The blood vascular system is here well developed, 

 and in all its parts shows in a very marked manner the spongy 

 structure characteristic of Echinoderms {i.e. it consists of lacunar net- 

 works or plexuses). One lacunar network covers the axial organ, and 

 another spreads out over the intestinal wall. Both are in open com- 

 munication with a lacunar plexus which surrounds the oesophagus, and 

 which may, at one point, become differentiated into a blood gland 

 (spongy organ) by the deposition of numerous blood formative cells in 

 its meshes. 



XI. The Nervous System. 



This system in the Echinodermata is developed in a quite peculiar 

 manner, unknown in other animals. It is composed of three alto- 



