ix PHYLUM ECHINODERMATA 417 



ambulacral areas and giving off branches to the ampullae of the 

 tube-feet or to the tentacles. In most of the Holothuroidea 

 branches pass forwards to the circlet of shield-shaped or branched 

 oral tentacles, and in some cases there are vesicles or ampulla at 

 their bases. In the Apoda, in which tube-feet are wanting, radial 

 vessels are also absent, and the vessels to the tentacles come off 

 directly from the ring-vessel. In all the classes, except Crinoidea, 

 one or more bladder-like appendages the Polian vesicles are in 

 most cases connected with the ring-vessel. The racemose vesicles, 

 or Tiedemann's vesicles (p. 377), are characteristic of the Asteroidea. 

 In all, except the Crinoidea and the majority of the Holothuroidea, 

 there is a communication between the ring- vessel and the surround- 

 ing water through the madreporic canal. In the Asteroidea, and in 

 Cidaris among the Echinoidea, the wall of this tube is strengthened 

 by numerous calcareous ossicles. In the Asteroidea, Ophiuroidea, 

 and Echinoidea the communication with the exterior is through 

 the madreporite. The fine pores perforating the madreporite 

 and placing the madreporic canal in communication with the 

 exterior, and the madreporic canal itself, are lined with strong cilia 

 which move so as to drive a strong current inwards the effect 

 being to keep all parts of the ambulacral system in a condition of 

 turgidity. In the few Holothuroids in which such a communication 

 exists (Elasipoda) there is usually a simple opening, but sometimes 

 a number of pores crowded together. In the remainder of the 

 Holothuroidea the distal end of the madreporic canal, or canals, 

 lies free in the interior of the body-cavity, with which it is placed 

 in communication by a number of perforations. In the Crinoidea 

 there is no madreporic canal ; but the ring- vessel is placed in 

 communication with the ccelome by means of a system of ciliated 

 water-tubes, while the ccelome communicates with the exterior 

 through a number of minute water-pores, which perforate the 

 oral body-wall. The fluid contained in the ambulacral system 

 is similar to that in the coelome, and contains similar corpuscles. 

 In one Ophiuroid, however, the ambulacral system contains 

 corpuscles coloured red with haemoglobin. Tiedemann's vesicles 

 appear to have the function of manufacturing the corpuscles. 



It cannot be definitely stated that a blood-vascular system 

 exists in the Echinoderms. But two systems have been regarded 

 as playing the part of blood-vessels the perihcemal system and 

 the hcemal system. Neither of these systems comprises vessels 

 with contractile walls, and there is no definite circulation of the 

 contained fluid. The perihsemal or, as it is sometimes termed, 

 pseudohaemal system, is present in all the classes of the phylum. 

 When typically developed (Asteroidea, Ophiuroidea) it consists of 

 a ring-like circum-oral vessel or sinus and five radial vessels given 

 off from it, together with an axial sinus and aboral ring-vessels. 

 These " vessels " are channels with a definite epithelial lining, 



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