448 COMPARATIVE ANATOMY CHAP. 



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

 body cavity, and at one point of the pseudoheemal ring, in the maclre- 

 poritic interradius with the axial sinus. 



The pseudohaemal system, in the Ophiuroidea and Asttroidea, is 

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

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

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

 thelium. Such a lining, in the case of a schizoccelomic 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 Holothurioidca the oral pseudoha-mal ring is, in the Paractinopoda 

 (Synapttdce), separated from each of the radial pseudohrcmal canals by a septum. 

 The pseudoluemal 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 pseudohremal ring is said to be wanting. The same is the case with the 

 well-developed radial pseudoheemal canals of the Echinoidca. In the Criiiid>:<i the 

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

 authors. The pseudohremal canals of the o^,//, /',///,/,,> give off lateral branches at 

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

 canal), and open into it. In the Axtt'r<ii<l> , 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 pseudohamial 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 pseudohfemal 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 pseudohaemal 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 comer 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 pseudohremal ring, and through this into the septum of the radial pseudo- 

 haemal 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 schizoccelomic nature of the pseudohsemal canals. 



IX. The Epineural System. 



In the Holothurioidea, the Echinoidea, and the Ophiuroidea 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 pseudohaemal 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 



