1 24 



Introduction to Animal Morphology. 



plates or spicules : this is called the perisome, 

 and it often bears spines. The nervous system 

 consists of a circum-oral filament, uniting the long, 

 radially-divergent threads, which are coloured orange, 

 brown, or green, by a pigment layer over the neuri- 

 lemma. The ciliated digestive canal is separate from 

 the coeloma or perivisceral cavity,* which is also 

 ciliated, and contains sea-water. A peculiar set of 



i i-. 18. 



Ambulacra! system of Echinus ; a, ambu- 

 5, sacculi ; r, circum-oral ring ; 



water - holding canals 

 <-xists (Fig. 1 8), separate 

 from the digestive tract, 

 although arising as an 

 outgrowth from it, lined 

 by ciliated protoplasm; 

 this consists of a ring 

 around the mouth, giving 

 off radial branches (c], 

 one or two in i-very anti- 



morp panVi acrnin fri\nnrr 



mere, eacn again giving 



Off rOWS Of Small, hollow, *> -l* = "* *dn*orifoni. plate. 



muscular processes (ambulacra, or pedicelli a), which 

 project on the surface, and by which the animal 

 moves. This connexion of locomotion with the water- 

 vascular system is characteristic. Attached to the 

 circum-oral canal are also inter-radial sacs (Polian 

 vesicles, Fig. 1 8, /), acting as reservoirs for the fluid 

 'which is sea-water containing chyle corpuscles). 

 Igvssiz compares this system with the canals and 

 tentacles of Ccelenterata. 



Muscular tissue is developed in the perisome in 

 inverse proportion to its calcification ; least in Echini, 



* But thi>, as well as the water vascular system, is an outgrowth from 

 the alimentary canal (Metschnikoff VD& A. Agassiz). 



