214 



PHYLUM ECHINODERMATA. 



consists in the fact that the ambulacral plates near the apex are 

 narrow, that they gradually become wider as they recede from 

 the apex, and then again become narrow as the ambitus is 

 approached (Figs. 152, 153). The pores of a pair, which are 

 placed near the outer sides of the plates and are connected by 

 grooves (yoked pores], accentuate this peculiarity and assist in 

 giving each row of ambulacral plates, in its part next the apex, 

 the appearance of a lanceolate leaf ; hence the term petaloid 

 which is applied to ambulacra modified in this way. In the 



FIG. 153. Test of an irregular sea-urchin of the Spatangid group, Brissopsis lyrifera (from 

 Claus). a from the aboral ; b from the oral side. In a the four genital pores are shown 

 (the posterior pore being absent), and the madreporite in the posterior interradius, in which 

 the anus .A/ is also placed ; the anterior ambulacrum has not undergone the petaloid modifi- 

 cation. In b the transversely elongated mouth near the front end, the three short ambu- 

 lacra of the trivium and the long ambulacra of the biviuin are shown, also the fact that the 

 peristomial plates of the interambulacra are single. 



Clypeastroids (Fig. 152) all the ambulacra present this modifica- 

 tion and constitute a five-leaved rosette, whereas in most 

 Spatangoids, when the modification is present, it is confined to 

 four ambulacra (Fig. 153), the anterior ambulacrum not sharing 

 in it and being different from the rest. The tube-feet issuing 

 from the yoked pores are said to be respiratory * in function. 



* This is the view of J. M tiller. It is based no doubt upon the double 

 connexion of the foot and its ampulla (p. 232). Inasmuch as the water- 

 vascular system is not distributed to the internal organs and therefore 

 cannot directly be of any use in supplying them with oxygen, it is perhaps 

 permissible to question whether these petaloid feet are correctly described 

 as respiratory. It is possible that they may have some quite different 

 function, e.g. that of sensation. 



