THE ASTERIDEA. 479 



radial canal, short branches are given off, which pass between 

 the ambulacral ossicles, and each opens into the neck of a 

 relatively large sac, with muscular walls (ambulacral vesicle), 

 which lies on the aboral face of the ambulacral ossicles in 

 the interior of the ray. The neck of the ambulacral vesicle 

 passes in the opposite direction into one of the pedicels. 

 Thus the ambulacral vessel communicates with the cavities of 

 all the pedicels on the one hand, and with the cavity of the 

 circuraoral ambulacral vessel on the other. Five pairs of 

 small eminences, consisting of caeca, which open into the cir- 

 cumoral vessel, are seated upon it ; and from one part of it, 

 opposite one of the interradial falciform folds already men- 

 tioned, springs a canal, which, taking a sinuous course, passes 

 to the aboral face, and terminates beneath the madreporic 

 tubercle ; this is the madreporic canal. It is not a simple 

 tube, but, as Sharpey first observed, its walls are doubly in- 

 voluted so as partially to obstruct its cavity, and it is strength- 

 ened by annular calcifications. The pores of the madreporic 

 tubercle place the cavity of the madreporic canal in commu- 

 nication with the exterior, whence it follows that the cavities 

 of the whole ambulacral system must be directly accessible to 

 the sea-water in which the Star-fish lives. The madreporic 

 canal is invested by the lining membrane of the peritoneal 

 cavity. This incloses a sinus, whicli accompanies the madre- 

 poric canal, and into the interior of which a fold projects. 



There is no great difficulty in ascertaining the existence of 

 the structures which have now been described, and all anato- 

 mists are agreed as to the nature of the ambulacral system. But 

 whether the neural canals are to be considered as a special 

 system of blood-vessels, and the sinus which accompanies the 

 madreporic canal, a heart, as is usually assumed, appears to 

 me to be very doubtful.^ I am disposed to think, in fact, 

 that not only these canals, but the circular, or rather pentag- 

 onal, vessel which has been described as situated on the abo- 



1 Since Tiedemann's time, the presence or absence of a blood-vascular sys- 

 tem in the Star-fishes has been alternately asserted and denied. The recent 

 investigations of Greef, " Ueber den Bail der Echinodermen" (" Marbm-g 

 Sitzungsberichte," 1871-'72), Hoffmann [l. c), and of Teuscher, " Beitrage zur 

 Anatomic der Echinodermen" {JeaaUche Zeltschrlft^ Bd. x. ), are in favor 

 of the existence of the " anal ring," and of an extensively ramified system of 

 canals, connected with it and with the neural canals. But it does not appear 

 to me that the fiicts, as they are now known, justify the assumption that these 

 canals constitute a distinct system of blood-vessels. Injections show that all 

 these canals communicate with the ambulacral vessels, and with the exterior, 

 by means of canals in the madreporic tubercle which open partly outward, 

 partly into the madreporic canal, and partly into the sinus which accompa- 

 nies it, and commmiicates with the circumoral neural vessel. 

 31 



