228 PHYLUM ECHINODERMATA. 



of which, with the aid of muscles and ligaments, the teeth are 

 brought into action. 



The perignathic girdle, which is absent in Spatangoida consists of pro- 

 cesses inwards of the ambulacral and interambulacral peristomial marginal 

 and sometimes adjacent plates. It is said to be continuous when the 

 ambulacral processes turn towards one another and unite so as to form 

 an arch the auricle through which the radial water-vascular canals and 

 nerves, etc., pass. The auricles are connected by the ridges which are 

 processes of the interambulacral plates. The girdle is said to be inter- 

 rupted when the ambulacral processes are absent or very small, while the 

 interambulacral processes are tall and diverge from one another, in such 

 a way as to tend to approximate over the ambulacrum ; when they touch 

 and form an arch over the latter, they constitute a false auricle. These 

 processes are for the attachment of the muscles of the jaws. They 

 have been compared to the ambulacral ossicles of Asteroidea. 



The body is covered with a ciliated ectoderm, which extends 

 over the smaller spines, the clavulae, sphaeridia and pedicellariae, 

 and on to the bases of the larger spines. Beneath it is the dermis 

 which contains pigment cells and a nerve-plexus, and the plates 

 of the skeleton. Inside the dermis is the ciliated peritoneal 

 epithelium. 



Muscles are not present in the body-wall, except in those forms 

 with flexible shell plates in which there are five pairs of longitud- 

 inal muscles, running meridionally within the test. 



Nervous system.* The arrangement of the ventral nervous 

 system is very similar to that found in Ophiurids. It is removed 

 from the surface, both the circumoral ring and the radial nerves 

 being contained in the epithelial wall of an epineural canal 

 (Fig. 169). It is connected by nerves, which pass through the 

 pores for the tube-feet, with the general sub -epithelial plexus of 

 the ectoderm (7, 8). The epineural canal is developed in the 

 larva by the closure of an ectodermal groove. 



The circumoral ring, which is connected with a sub -epithelial 

 or epithelial plexus over part of the intestine, lies between Aris- 

 totle's lantern and the peristomial membrane, close to the mouth. 

 The radial trunks lie between the epineural and perihaemal 

 canals, and give off nerves to the tube-feet, etc., and as stated 

 above to the integumentary plexus. They end by passing on to 



* For the physiology of the nervous system see J. v. Uexktill, Zeit. 

 Biol. (2), 21, 1899, p. 73, and 22, p. 447 ; also the article Echinodermata in 

 the Cambridge Natural History, p. 519. 



