446 COMPARATIVE ANATOMY chap. 



trades at certain points tlirougli the wall of the axial sinus into the general body 

 cavity. 



2. Ophiuroidea. — The axial organ is developed out of the wall of the sinu.s which 

 accompanies the stone canal on the side which is turned towards the periphery of 

 the disc ; and, further, out of that portion of the wall which is in contact with the 

 stone canal. It projects as a somewhat massive body into the sinus, occupying 

 almost its entire lumen [cf. Fig. 361, 8, p. 422). 



3. Echinoidea (Fig. 358, 32, p. 419). — The axial organ lies in the axial sinus, 

 which it almost completely fills, and to the wall of which it is attached bj' means of 

 numerous strands. It sends off a process into the sinus which lies below the raadre- 

 porite near the ampulla, this process perforating the wall which divides this sinus 

 from the axial sinus. 



4. Crinoidea (Fig. 384 gp, p. 482). — The axial organ, which is in this class 

 known as the genital stolon (or the glandular organ or the dorsal organ), is here 

 differently constructed. It originates as a thin strand in the axis of the chambered 

 organ, then ascends direct through the axial section of the body cavity of the calyx 

 towards the mouth, widening in the first part of its course, and then again narrowing. 

 It consists of a complex of much-twisted canals with narrow lumina, which are en- 

 closed in a stroma of connective tissue ; the lumina of these canals may disappear, 

 and they may become strands. They are lined with cylindrical epithelium. In the 

 axis of the chambered organ, the axial organ consists merely of a few very thin strands 

 or canals, bat when it leaves the chambered organ and ascends into the body cavity, 

 the canals swell and branch, so that their number increases till the middle of the body 

 cavity is reached. Then their number again decreases, one canal after the other 

 ending blindly. Finally, in the oral region, the axial organ consists of only a few 

 strands, which most probably are continued into the genital tubes or strands of 

 the arms. Such a connection has at least been demonstrated in the young Antedon. 

 and it has been further found that, ontogenetically, the genital tubes bud out of the 

 axial organ. 



G. The Chambered Sinus of the Crinoidea and Its continuation in 

 the Stalls and in the Cirri. 



Quite in the apex of the calyx, and in Antedon enclosed in the 

 centrodorsal, there is a cavity containing the apical parts of the 

 axial organ. This cavity is of enterocoelomic origin. It is divided, 

 by means of five radially arranged partitions of connective tissue, into 

 five chambers, which are covered on all sides by epithelium. This is 

 the "chambered organ" (Fig. 384 cli, p. 482). 



In stalked Crinoids the chambered sinus is continued into the 

 stalk, forming in it a canal. This is also divided, by five radially 

 arranged partitions, into five sub-canals (the continuations of the five 

 chambers of the sinus) arranged round a common axis. This common 

 axis is probably formed by a continuation of the axial organ. 



In each of the whorl joints of the stalk in those Crinoids which 

 have cirri, the five-fold canal widens to form a kind of repetition of the 

 chambered sinus, giving oflf into each of the cirri a lateral canal, which 

 runs through its whole length, and is divided into an upper and a lower 

 canal by a horizontal partition reaching inwards as far as to the 

 common axis of the stalk canals. 



