ECHINODERMA GENERAL DESCRIPTION 



of the Pentactcea, together with epithelial nerves on the floor of 

 the arm -grooves, as in Pelmatozoa. The Ophiuroids are, as 

 explained under Stelleroidea, scarcely to be distinguished from 

 Asteroids. Whether they branched off at an earlier date or no is 

 uncertain ; at any rate, they have progressed farther from the 

 Pentactcea type, in so far as the radial nerves have sunk below 

 the surface and are covered by " epineural canals," which probably 

 represent closed food-grooves (Fig. XIII. 1 and 2). 



Fio. XIII. 



Sections across ambulacra of 1, Asteroid ; 2, Ophiuroid ; 3, Echinoid ; 4, Holothurian. 

 amb, ambulacral ossicle ; amp, ampulla ; b, radial blood-vessel ; cm, circular muscles ; d.s, 

 ventral scute ; e, radial epineural canal; Im, longitudinal muscles ; l.s, lateral scute ; m, muscles ; 

 n, radial nerve of superficial oral system ; w2, radial nerves of deeper oral system ; p, podium ; 

 ph, pseudhaemal canal ; v.o, vertebral ossicle ; w, radial canal of the water-vascular system. 



The development of Echinoidea has been studied by J. Miiller 

 (1852), Agassiz (1864), Metschnikoff (1869), Bury (1889), and 

 many others. The results are summarised by Theel in his admir- 

 able account of the development of Echinocyamus pusillus (1892). 

 Up to the stage corresponding to the Dipleurula no important 

 divergences are manifest. The peculiarities of the ensuing meta- 

 morphosis appear due to the extreme development of a free- 

 swimming Pluteus (Fig. XIV.). At an early stage there is an 

 invagination (am) of the ectoderm on the left side between the 

 bases of the ventral and dorsal posterior processes of the Pluteus. 

 The inner end of this sac grows towards the left hydrocoel, 

 while its opening nearly or quite closes (Fig. XV. 1). The five 

 primitive lobes of the hydrocoel grow up into the floor of this 

 sac (Fig. XV. 2), which thus serves as a kind of amnion in which 

 the young sea-urchin is formed (Fig. XV. 3), until the size of the 



