124 



lutrodtidioii to Animal Morphology. 



Fit:. 1 8. 



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 

 water - holding canals 

 exists (Fig. 18), 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 every anti- 

 mere, each again giving 



off rows of small, hollow, ^' P"l'^°-vesicle ; m. madreporiform 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, p), acting as reservoirs for the fluid 

 (which is sea-water containing chyle corpuscles). 

 Agassiz compares this system with the canals and 

 tentacles of Coelenterata. 



Muscular tissue is developed in the perisome in 

 inverse proportion to its calcification ; least in Echini, 



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

 tlie alimentary canal [Metschiiikoff xad A. Agassiz). 



Ambulacral system of Echinus ; a, ambu- 

 lacra! pedicelli ; b, sacculi ; c, circum-oral ring ; 



