26 ECHINODERMA GENERAL DESCRIPTION 



placed between the ring and radial nerves of the oral system 

 above, and the ring and canals of the water-system below. This 

 system is dominant, perhaps the only one, in Stelleroidea, where 

 it communicates with the general body-cavity and the axial sinus ; 

 it is present in Echinoidea and Holothurioidea, in which classes it 

 is said to be closed ; it is so much reduced in the Crinoidea that 

 its existence is denied by some authors. In Asteroidea (Figs. XXL, 

 XXII.) the ring is divided into an outer and an inner ring by an 

 oblique septum, from each angle of which a vertical septum passes 

 down each radial canal. Formerly the system was supposed to 

 develop as a cleft in the mesenchyme, and therefore was called the 

 " schizocoelic system " ; MacBride (1896) has shown that in Asterina 

 the inner ring is an outgrowth from the axial sinus, while each of 

 the five compartments of the outer ring and canals arises separately 

 as an outgrowth of the coelom, the outgrowth in the madreporic 

 interradius being derived from the anterior coelom, the rest from 



the left posterior coelom. (b) Lacunar, 

 present in all classes except perhaps 

 Stelleroidea, and developed as lacunae 

 or small spaces in the connective tissue, 

 and therefore mesodermal (Fig. XXL). 

 It is differentiated into a network in the 

 wall of the gut, absorbing therefrom the 

 nutrient fluid, which is carried by a 

 main trunk on each side of the gut to a 

 circumoral ring; from this run radial 

 FIO. xxii. canals, below the pseudhaemal canals 



Diagram of the pseudi.aemai when present, and above the water- 

 system of Asterina (after MacBride). , A , ., ., . , . , 

 Seen from above: M marks the M VCSSelS, While it IS Connected With net- 

 plane; in and ex, inner and outer wnr l,. Q nn t l> A filir fj, p n f k n fV, annnrls nnH 

 divisions of the perihaemal ring ; a, WOrKS On tne SUHaCC 01 DOtn gOnaQS and 



the one arising from the anterior ax j a i organ. The lacunar system of the 



coelom; ox, axial sinus, passing <>,,, ., , ._, . . , 



towards, but not opening into, the btelleroidea diners in the absence of an 



absorbent network, and is, says Cuenot, 

 a derivative of the axial organ, and 

 therefore endodermal, i.e. it is only the pseudhaemal system greatly 

 extended. 



Respiration takes place through all exposed processes of the 

 ambulacral system, and through the body wall where thin enough, 

 as in some Holothurians. Specialised outgrowths or foldings of 

 the latter are : the " external gills " of Echinoidea, outgrowths of 

 the circumoesophageal sinus ; the papulae of Asteroids, containing 

 diverticula of the body cavity; the bursae of Ophiuroids ; the "pectini- 

 rhombs " of some Cystids ; the " hydrospires " of Blastoids and 

 some Crinoids. Respiration is also effected by water entering the 

 alimentary canal, whether through mouth or anus ; in the latter 

 case it is again expelled. Special structures connected herewith 



