ix PHYLUM ECHINODERMATA 393 



it contains numerous strands of connective tissue. Special 

 organs providing for the respiration of this fluid are the dermal 

 branchial or papula:, the Stewart's organs and the respiratory 

 trees. The first of these, which are confined to the Asteroidea 

 and Echinoidea, have been described in the accounts of the 

 Starfish and Sea-urchin. In most Asteroidea they occur only on 

 the dorsal surface, but in some forms they are present on the 

 ventral surface as well. In some of the Echinoids the place of 

 dermal branchiae in providing for the respiration of the compart- 

 ment of the ccelome between the peristome and Aristotle's lantern 

 is taken by Stewart's organs, arborescent bodies which project 

 inwards from the peristome. The respiratory trees are referred 

 to below in connection with the enteric canal. 



Some reference has already been made, in describing the general 

 form of the body, to the ambulacral system of vessels. A 

 ring-like circum-oral vessel (ring-vessel) in nearly all cases sends off 

 a series of radial branches, one passing along each of the rays or 

 ambulacral areas and giving off branches to the ampulla; 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 

 the 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 con- 

 nected with the ring- vessel. The racemose vesicles, or Tiedemanns 

 vesicles (p. 354), are characteristic of the Asteroidea. In all, 

 except the Crinoidea and the majority of the Holothuroidea, there 

 is a communication between the ring-canal and the surrounding 

 water through the madrcporic 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 ; in the few Holothuroids in which such a com- 

 munication 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 madre- 

 poric canal, or canals, lies free in the interior of the body-cavity, 

 with which it is placed in communication by a number of per- 

 forations. 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 com- 

 municates with the exterior through a number of minute ivater- 

 jiiu'i's, which perforate the ventral body- wall. The fluid contained in 

 the ambulacral system is similar to that in the ccelome and contains 

 similar corpuscles. In one Ophiuroid, however, the ambulacra! 

 system contains corpuscles coloured red with haemoglobin. 



