THE ECHINOIDEA 299 



the mouth downwards ; as its gonads were quinqueradiate ; and as 

 its ambulacra extended from the peristome almost to the aboral 

 pole. The union of these three characters separates the Echinoidea 

 from the rest of the Echinoderma. 



Proceeding to discuss the Sub-Classes, Orders, and Families of the 

 Sea-urchins, we may sum up their common characters in the following : 



Diagnosis of the Class. 1 The Echinoidea are eleutherozoic Echino- 

 derma which are actinogonidial (i.e. having the gonads quinqueradiate) 

 and zygopodous (the podia extending from the peristome to near the 

 aboral pole). The body is covered by numerous series of plates, usually 

 polygonal and in vertical series. The apical system may be absent, 

 rudimentary, well developed, or very extensive. The gonads are un- 

 paired and interradial. The body is spherical, or flat, or bilaterally 

 symmetrical, and is covered by spines which may be long, stout, and 

 strong, or present every stage of reduction to such as are fine and 

 silky. An anus is always present, but its position is variable ; but 

 it is either at the aboral pole or in the posterior interradius. Respiration 

 is partly by gills and partly by podia. Development is either direct or 

 indirect. 



The usual primary subdivision of the Echinoidea is into two sub- 

 classes PAL^EECHINOIDEA and EUECHINOIDEA ; the former including ap- 

 proximately all the Palaeozoic, and the latter all the Neozoic Echinoidea. 

 The last formal attempt to define the two groups was that of Duncan 

 (24, p. 4), which has been accepted by Jackson (41). 



Excluding from Duncan's diagnoses characters common to the two 

 sub-classes, we find that the only distinction between them is that the 

 Palseechinoidea have either one or more than two vertical rows of plates 

 in each interambulacrum, and two or more vertical rows of plates in each 

 ambulacrum ; while the Euechinoidea have two vertical rows of plates in 

 each interambulacrum and in each ambulacrum. 



This classification is open to two fatal objections. The rule is not 

 absolute. Thus in the Cretaceous genus Tetracidaris there are four rows 

 of plates in each interambulacrum ; and in such genera as the Euechinoid 

 Tripneustes, it is no more correct to say that there are only two rows of 

 vertical plates in the ambulacra, than it would be to say so of the Palae- 

 echinid Palceechinus which is described as having more than two vertical 

 series. In the second place, the classification separates fairly close allies, 

 and brings together extremely divergent forms. Thus, such a species as 

 the Liassic Euechinoid Gidaris edwardsi is far more closely allied to such 

 a Palseechinoid as Archaeocidaris than the latter is allied to Tiarechinus. 



The separation of these two sub-classes was originally based on several 

 very definite characters, such as the imbrication of the plates, the flexi- 

 bility of the test, and the number of pores in the genital plates. One by 

 one these characters have been shown to be valueless for the purpose for 

 which they were used, but the classification based on them has been re- 

 tained. It is preferable to return to the division of the Echinoidea into 

 REGULARIA and IRREGDLARIA. 



1 Emended from Bell (10), p. 214. 



