554 THE INVERTEBRATA 



The nervous system consists of plexuses of fibrils and nerve cells 

 underlying various epithelia and thickened in places into denser 

 tracts (" nerves "). It is remarkable not only for remaining in this 

 primitive condition, but for being partly derived from mesodermal 

 epithelia. It is in three parts : (i) the ectoneural system underlying the 

 w^hole ectoderm as a plexus (see p. 550) and thickened {a) along each 

 ambulacrum as a radial nerve, (b) around the mouth as a nerve ring, 

 which connects and has been found by experiment to co-ordinate the 

 radial nerves (a and b are, with an epithelium, removed from the 

 surface of the body save in asteroids and crinoids), (c) as branch nerves 

 to such structures as tube feet, spines, etc. ; (2) the deep oral system 

 underlying the mesodermal epithelium of the perihaemal vessels and 

 having a distribution similar to that of the ectoneural system but less 

 extensive than the latter and in particular defective in the Echinoidea ; 

 (3) the aboral or apical system, also mesodermal in origin, developed 

 from the peritoneum on the aboral side. This system is best developed 

 in the Crinoidea, where it is removed from the general peritoneum 

 and enclosed in the ossicles. Here it has the form of a nerve along 

 each arm and a complex central station in the "chambered organ" 

 (see below). In the Asteroidea it runs as a cord above the peritoneum 

 of each arm, the cords meeting in the middle. In the Ophiuroidea 

 and Echinoidea it is a ring in the aboral sinus. It is not found in the 

 Holothuroidea. The mesodermal nervous systems are principally 

 motor, innervating the muscles which move the internal skeleton. 



The Echinodermata are poorly provided with sense organs. There 

 is a general sensitiveness of the epithelium of the body, at least to 

 tactile stimuli, which is heightened in the podia and in the terminal 

 tentacle which stands at the end of each radial water vessel in the 

 Asteroidea, Ophiuroidea, and Echinoidea. The olfactory sense is 

 perhaps also located in the podia or in some of them, especially in 

 those that are situated around the mouth and in the Holothuroidea 

 are developed into tentacles. An eye-spot is situated at the base of 

 each terminal tentacle in the Asteroidea, and certain holothurians 

 possess statocysts in the skin. 



All echinoderms are marine in habitat. Few of them are pelagic : 

 none are parasitic. Only the Crinoidea are fixed, and some of these 

 are only temporarily so. 



The majority of members of the phylum have free, pelagic larvae', 

 though some, as Asterina, pass a considerable time in the egg mem- 

 brane and have larvae which are not pelagic ; and a few, chiefly polar 

 or deep-sea species, keep the young in brood pouches until they have 

 the adult form. The eggs of the species which possess pelagic larvae 

 are small ; the others larger and more yolky in proportion to the late- 

 ness of the stage at which they are set free. Fertilization takes place 



