566 INVERTEBRATE MORPHOLOGY. 



in the walls of the oesophagus in the Crinoids. They have 

 been termed blood-vessels in the Ophiuroidea, the sinuses 

 which surround them being termed the perihsemal canals; 

 they follow the course of these latter, a process of the ovoid 

 gland coming into connection with the oral lacunar ring. 

 This gland (Fig. 259, 6) is, as in other groups, partly asso- 

 ciated with the lacunar system and partly with the genital 

 apparatus. It lies in the wall of the axial sinus and projects 

 into it so as almost to fill it. At one extremity, as stated, it 

 comes into connection with the oral lacunar ring, and at one 

 point in its wall it contains a mass of cells from which the 

 genital cords pass out to the reproductive organs, accom- 

 panied by strands of the lacunar tissue. 



The hydrocoel has the usual arrangement, and is confined 

 to the oral surface of the disk and arms. The radial canals 

 (Fig. 260, lur) lie on the oral surface of the ambulacra! ossi- 

 cles, extending to the terminal plate, and ending, at least in 

 those forms which have simple arms, in a terminal tentacle. 

 At regular intervals, corresponding in number to the ambu- 

 lacral ossicles, the radial canals give off transverse branches, 

 which pass outwards in the substance of the ossicles (Fig. 258), 

 and make their exit through the ambulacral pores between 

 successive adambulacral plates to terminate as tube-feet. No 

 ampullae occur on these transverse branches, though a circu- 

 lar valve occurs just where each branch becomes continuous 

 with the tube-foot. The feet are simple conical structures 

 destitute of a terminal sucker, and do not therefore serve 

 for locomotion. Their walls are richly supplied with nerves, 

 and in some forms are provided with numerous papillae ap- 

 parently sensory in function. Surrounding the mouth are 

 ten buccal tentacles (Fig. 260, ht), which correspond to the 

 first two pairs of tube-feet of each radius of the Asteroids, 

 but arise by fine bi'auches, which later divide, and are 

 directly connected with the oral ring-canal (Fig. 258, i). 

 These seem to be undoubtedly sensory and perhaps olfactory 

 in function. The oral ring-canal usually has attached to it 

 in each interradius, except that in which the stone-canal lies, 

 a single Polian vesicle (Fig. 260, PV), though in Ophiactis 

 two, three, or even four vesicles may occur in each interra- 



