298 THE ECHINOIDEA 



apical system is exocyclic, the jaws become less important. In 

 the Clypeasters the jaws are massive, but their power of move- 

 ment slight, as they are poised on small vertical processes which 

 fit into a socket on the pyramids. 



In the Holectypina we see a gradual reduction of the peri- 

 gnathic girdle, the processes becoming low and the ridge important 

 (as in Discoidea), until in Galerites the ambulacral processes are 

 absent, and there are only five low interradial thickenings which 

 act as ridges. Finally, from the Atelostomata the perignathic 

 girdle and jaws are completely absent. 



The Generative Glands are fairly constant in character, but their 

 number varies, one or more being lost in many Irregular Echinoids. 

 The young are generally free-swimming plutei, but some species 

 are viviparous, the young being nursed in marsupial depressions 

 (Fig. XLIIL, see Wyv. Thomson, 81). 



In spite of these great variations in structure there are several 

 characters common to all Echinoids. By the selection of those 

 structures which are found in all the Echinoids (except when they 

 have been lost by obviously secondary modification) it is possible to 

 conceive a schematic Echinoid. This is useful, as it helps us to an 

 idea of the primitive ancestor of the class, and as it brings into pro- 

 minence the features which separate the Echinoidea from the other 

 Echinoderma. 



The Primitive Echinoid for which various names have been 

 suggested probably had a globular muscular body, covered by an 

 irregular series of polygonal plates. It must have had a simple 

 alimentary canal rising from a mouth situated at the centre of the 

 lower surface ; at first, possibly, it may not have had an anus, 

 which, when it came, opened on the upper surface. Three rings 

 surrounded the oesophagus, and from each ring five branches passed 

 up the test to the aboral pole. These three rings with their 

 branches formed the water-vascular, blood-vascular, and nervous 

 systems. Branches from the radial vessels of the water-vascular 

 system passed between the plates of the skeleton to the exterior 

 and acted as suckers. The suckers, by absorption of parts of 

 the plates, .at length passed out through pores, instead of through 

 the sutures. The perforated plates were therefore marked off 

 from the others and formed the five ambulacra, while the imper- 

 forate plates between constituted the interambulacra. A tube 

 connected the water-vascular ring with the exterior, and allowed 

 the entrance of the necessary water by a single pore. Five un- 

 paired gonads occupied the interambulacral areas. The primitive 

 Echinoid did not have either a stalk, apical system of plates, 

 masticatory apparatus, or perignathic girdle. 



Such an animal would have been regarded as an Echinoid, as it 

 was not attached by the aboral surface, but on the contrary had 



