TYPE ECHINODEBMA. 539 



contained in flat non-nucleated disks, resembling Mammalian 

 red blood-corpuscles, floating in the plasma of the water 

 vascular system ; in the Holothurians, Thyone and Cucumaria, 

 it is, however, contained in amoeboid corpuscles, which are 

 most abundant in the coelomic fluid, though occurring also in 

 the water vascular tubes. 



The digestive tract is generally more or less twisted into a 

 spiral ; and even when, as in some Holothurians, it appears to 

 be straight, it is to be regarded as a much-drawn-out spiral, 

 since the mesentery still retains a spiral arrangement. In the 

 Holothurians, Echinoids, and Starfishes it opens on the aboral 

 surface of the body, but in the Crinoids it is bent upon itself 

 so that the anus is on the oral surface. In some Starfishes 

 and in all Ophiurans no anus is present. Yarious accessory 

 structures, masticatory apparatus, coecal pouches, etc., are 

 found in the various groups, but their description may be 

 deferred until later. 



The nervous system may be regarded as being composed of 

 three portions, one of which has essentially the same arrange- 

 ment as the water vascular tubes, consisting of a circumoral 

 or a perioesophageal ring from which five radial nerves pass 

 off (Fig. 265, nr and rn). In the Starfishes and Crinoids the 

 entire system is imbedded in the ectoderm, but in other forms 

 it sinks within the body-cavity. From it branches pass in- 

 wards at the mouth to supply the walls of the oesophagus, and 

 other branches form a network covering the surface of the 

 body, supplying the sense-organs which may occur thereon. 

 The radial nerves, in addition to sending branches to join the 

 epidermal plexus, supply the ambulacral system. This por- 

 tion of the nervous system may be termed the epidermal 

 portion, and the second, inasmuch as it supplies the majority 

 of the muscles of the body, may be termed the muscular 

 system. This is not always developed, being absent in the 

 Crinoids, but when present accompanies in general the epi- 

 dermal portion in the form of delicate nerve-cords lying on 

 the inner surface of the circumoral ring and radial nerves, and 

 sending branches to the various muscles, including possibly 

 those of the tube-feet. The third or aboral portion appears 

 to be entirely wanting in the Holothurians, but when present 



