THE CRIXOIDS. 



275 



vessels, on their upper or ventral side, are three tubular prolongations of the body-cavity. The middle 



one of these contains the branched generative gland (Fig. 21, ov); while a current of water due to 



ciliary action proceeds outwards along the arms by the upper canal, and returns to the disc by the 



lower one. 



The blood- vascular system of a Crinoid is considerably more complicated than that of the 



Echinozoa, owing to the presence of organs that are altogether unrepre- 

 sented in that group, and are connected with the development of the 



stalk which all Crinoids possess for a longer or shorter period of their life. 



Situated more or less exactly in the vertical axis of the disc is a tabulated 



organ, the central plexus (Fig. 20, c-p). This corresponds to the similarly 



named organ in the Eckinozoa (Fig. 10, cp), and consists of a bundle 



of blood-vessels. Some of these terminate above in the oral blood- vascular 



ring (Fig. 20, ob), first traversing an extensive network (Fig. 20, Ip) 



which is situated in the lip immediately below. Others extend outwards 



beneath the food-grooves of the disc (Fig. 20, ag) into the rays and arms, 



and surround the genital glands (Fig. 21, gv). Others, again, give off 



side branches, which form a network over the digestive tube (Fig. 20, ib). 



Towards the bottom of the disc the vessels of the central plexus, instead 



of joining into an aboral ring, group themselves into an inner set sur- 

 rounded by five outer ones, which correspond in position with the radiala. 



They pass downwards through the central funnel between the inner ends 



of the first radials, at the bottom of which the five outer vessels expand 



into five large chambers, which are regularly arranged around the central 



vascular axis. The structure thus formed, which is known as the 



" chambered organ " (Fig. 20, c/t), is lodged within the cavity of the 



centrodorsal piece (cd). It is enclosed in a fibrillar envelope, processes 



of which extend outwards through all the joints of the rays and arms 



(Figs. 20, 21, a), and also into the cirri (ci), or clawed hooks borne upon 



the centrodorsal. These extensions into the cirri lodge minute blood- 

 vessels (Fig. 20, civ), which are continuous either with one of the 



chambers of the chambered organ or with one of the vessels in its 



central axis. In the Stalked Crinoids 

 the chambered organ is contained 



within the calyx, and the chambers are continued down the 

 central canal of the stem, as five vessels enclosing a core of 

 smaller ones. When the stem bears whorls of cirri, as in Penta- 

 crinus (Fig. 18), the five outer vessels expand slightly in each 

 cirrus-bearing joint, and each gives off one cirrus-vessel, the whole 

 forming a small edition of the chambered organ in the calyx above. 

 The course of the extensions into the rays and arms of the 

 fibrillar envelope of the chambered organ, which are known as 

 the axial cords, is seen in Fig. 22. It is very difficult to deter- 

 mine whether they enclose blood-vessels, as the axial cords of 

 the cirri do, but they are of extreme importance in another way. 

 For all the movements of the arms and pinnules depend upon 



if- the integrity of their axial cords, and upon the connection of 



Fig. 22. DIAGRAM SHOWING THE COURSE , , . , 1jC 1 I, U J 



OF THE AXIAL CORDS PROCEEDING FROM these cords with the central fibrillar envelope or the chambered 

 THE CHAMBERED ORGAN WITHIN THE or <r an . Some of the extensive branches (Fig. 21, a ') which are 



CALYX OF A FEATHER-STAR. (After H , f xl 1 1 j. 



Ludwig.) ' given off from the axial cords within every joint of the skeleton 



a> s?cond > ^na ; th B iVd fl i' : adiar s m " :ioint; B1> ^ **' flvst> are distributed to the muscles connecting the successive joints 



(Fig. 20, am). 



The nervous apparatus beneath the food-grooves (Fig. 21, n) is not connected with the muscles, 

 ar-d has no influence whatever upon the movements of the skeleton, which will continue to swim 



Fig. 21. CROSS-SECTIOX OF A 

 PINNULE OF THE ARCTIC 

 FEATHER-STAR (Comatula es- 

 rfirtcW.it), MAGNIFIED SEVENTY- 

 FIVE TIMES. (Slightly altered 

 from H. Ludwig.) 



a, axial cord ; a', its branches ; 03, arabu- 

 lacral or food groove ; b, radial blood- 

 vessel; gv, genital vessel; n, radial 

 nerve; on, ovary ; pj, pinnule joint ; tc, 

 water-vessel ; T, tentacles. 



