448 COMPARATIVE ANATOMY CHAP. 



it communicates, by means of numerous apertures, with the general 

 body cavity, and at one point of the pseudohsemal ring, in the madre- 

 poritic interradius with the axial sinus. 



The pseudohsemal s}*stem, in the Ophiuroidea and Asteroidea, is 

 said to arise, ontogenetically, as a cleft in the connective tissue 

 (mesenchyme), and thus to be a sehizoecelomie structure. It is, 

 however (as has been proved in the Holothurioidea), lined with endo- 

 thelium. Such a lining, in the case of a sehizoecelomie cavity is, how- 

 ever, so incongruous, that we are justified in desiring further evidence. 

 (For the position of these canals, see Figs. 352-356, pp. 409-413.) 



Special. In the Holoth urioidca the oral pseudoh;emal ring is, in the Paractinopm 1, i 

 (Stjnaptidce), separated from each of the radial pseudohfemal canals by a septum. 

 The pseudohffiinal canals stretch only a short way backwards. In the Actinopoda, 

 they run the whole length of the body, but are said also to end blindly at both ends, 

 and the pseudohfemal ring is said to be wanting. The same is the case with the 

 well-developed radial pseudohfemal canals of the Ecldnoidca. In the Crinoidca the 

 canals are certainly very much reduced, their existence is altogether denied by some 

 authors. The pseudohsemal canals of the i^t/tini-oidea give off lateral branches at 

 regular segmental intervals ; these branches ascend to the brachial cavity (dorsal 

 canal), and open into it. In the Astcruidi:a, both the circular and the radial vessels are 

 divided into two by a longitudinal septum. In the radial canals the septum is vertical, 

 in the pseudohremal ring it runs slantingly, dividing it into an outer and lower and 

 an inner and upper canal. The latter, the inner and upper canal, in the madreporitic 

 interradius, communicates with the axial sinus ; the former, the outer and lower 

 canal, is in open communication with the body cavity of the disc by means of five 

 ascending, interradial, lateral canals. At regular intervals, between every two 

 consecutive tube-feet, each radial pseudohaemal canal is connected with two marginal 

 canals which run longitudinally at the edges of the ambulacral furrow. Each tube- 

 foot receives two canals from the pseudohwrnal system, which run to its tip ; one of 

 these comes from the radial canal, and the other from the lateral canal. The lateral 

 canal, further, sends off at each corner between two consecutive ambulacral plates 

 and the contiguous adambulacral plate, a lateral branch, which runs up between 

 these plates. This lateral branch opens into the brachial cavity. 



Two specially interesting facts deserve notice : (1) the mesentery by means of which 

 the axial organ is attached to the wall of the axial sinus is continued into the septum 

 of the pseudohfemal ring, and through this into the septum of the radial pseudo- 

 hfemal canals ; and (2) the axial organ, although in a reduced condition, may even 

 be produced along a greater or smaller portion of these septa. These facts throw 

 further doubt upon the sehizoecelomie nature of the pseudohfeinal canals. 



IX. The Epineural System. 



In the Holothurioidea, the E<-]iiii<>i<li, and the 0].>lt'h<r<>i<l<:n the oral 

 nervous system is accompanied by canals known as the epineural 

 canals, which run between it and the adjacent body epthelium. 

 This epineural system thus repeats on the outer side of the oral 

 nervous system the pseudoh?emal system which accompanies the 

 nerves on their inner side, and, like the latter system, it consists of 

 an oral circular canal, and radial canals. In the Asteroidea and 



