CRINOIDEA. 413 



cates by numerous water-tubes with the body-cavity, to which the 

 sea-water is freely admitted by minute pores in the body-wall ; 

 while some of these pores are said by Perrier to lead directly 

 into the oral water-vascular ring, though this is denied by Hamann. 

 The vascular system is extraordinarily developed, and its upper 

 portion consists of an oral ring, which gives rise to the radial 

 vessels, and is also connected with a peculiar organ apparently 

 serving as a kidney, as well as with numerous " intervisceral " 

 vessels. There is no aboral vascular ring, but the vessels become 

 connected inferiorly with a singular quinquelocular structure known 

 as the "chambered organ," which is contained within the centro- 

 dorsal plate of the calyx. The chambered organ is enclosed in a 

 peculiar fibrillar sheath, the nature of which will be spoken of 

 immediately, and it sends prolongations into all the arms, along 

 canals contained within the skeleton of the latter, and also into 

 the dorsal cirri. 



In the Crinoids generally, the structure of the vascular system is, up to 

 a certain point, much the same as it is in Coinatula. Occupying the 

 dorsoventral axis of the body is the lobated kidney, enclosed in a sheath 

 of vessels. Dorsally, these resolve themselves into a central group (of 

 one or more), and five peripheral vessels, the latter expanding in the 

 lower part of the calyx into the five chambers of the " chambered organ." 

 In the Pedunculate Crinoids the chambers narrow again, and the group 

 of vessels is continued down the central canal of the column. In Penta- 

 crinus, which has cirri at regular intervals, the five peripheral vessels ex- 

 pand in each cirrus-bearing joint into five dilatations, which thus give rise 

 to a miniature " chambered organ," each chamber of which gives off a 

 single vessel to a cirrus. 



The nervous system of the Feather-stars is also extraordinarily 

 developed as compared with that of the other Echinoderms. As 

 has been previously seen, there is found under the floor of each 

 of the brachial grooves a fibrous nerve-band (fig. 287, //), which 

 corresponds morphologically with one of the radiating nerve-fibres 

 of a Star-fish. In addition to these ambulacral nerves, the " cham- 

 bered organ," above spoken of in connection with the vascular sys- 

 tem, is enclosed in a peculiar fibrillated sheath, which has been 

 shown by Dr W. B. Carpenter to be of a nervous nature. It gives 

 off a series of radial prolongations or "axial cords" (fig. 287, a), 

 which occupy median canals within the skeleton of the arms and 

 pinnules, and send numerous branches to the muscles and the 

 integument (fig. 287, a). In the stalked Crinoids the fibrillar 

 nerve-sheath is likewise prolonged, along with the blood-vessels, 

 into the central canal of the column or peduncle. No represen- 

 tative of this peculiar system of nerves is known in the unstalked 

 Echinoderms (Echinozoa). 



