86 ECHINODERMATA. CRINOIDEA. 



The blood -vascular system has a similar distribution, the 

 radial vessels being immediately under the nerve cord ; 

 but there is in addition to this a bundle of blood vessels 

 situated in the axis of the disc and known as the central 

 plexus, this gives off branches to the oral ring and 

 elsewhere ; dorsally it is connected with what is known as 

 the chambered organ, which is placed in the centro-dorsal 

 plate, or sometimes between the basal plates of the calyx. 

 In addition to the nervous system already mentioned, 

 there is another placed dorsally, consisting of a fibrous 

 sheath surrounding the chambered organ, and of cords 

 given oft' from it, which pass through canals in the plates 

 of the calyx to the arms and pinnules. The fibrous 

 sheath is also prolonged into the stem when present and 

 contains in its centre a vessel from the chambered organ. 

 The water- vascular system consists of radial vessels placed 

 dorsally to the blood vessels and of a ring round the 

 mouth ; from the latter, tubes hang down and open into 

 the body-cavity, which communicates with the water of 

 the exterior by means of pores. In connection with the 

 radial vessels are tubular processes, the tentacles, which 

 form a row on each side of the food-grooves, and correspond 

 with the tube feet of the star-fish, but do not function in 

 locomotion. 



We come now to the structure of the skeleton. In 

 the majority of crinoids a stem is present, this is more or 

 less flexible and is sometimes several feet in length. It is 

 formed of a number of circular or pentagonal (occasionally 

 elliptical) plates, articulated by their broad surfaces. Each 

 plate is pierced in the centre by a canal which is circular 

 or pentagonal and contains a prolongation of the cham- 

 bered organ and its fibrous sheath. The articular faces of 

 the plates are often provided with radiating striae or with 



