NUMBER AND ENUMERATION OF RADII. 119 



be departed from.* In most cases the number of radii is deter- 

 mined early in development, but in a few forms (e.g. Labidiaster) 

 the number increases with the growth of the animal, at any rate 

 in the early stages of the adult (Perrier). In Cystids the number 

 of radii varies more than in any class, the number two or three 

 sometimes occurring. 



Enumeration of Radii. After some hesitation we have decided 

 to adopt a definite enumeration of the radii for all classes of 

 Echinoderms. Our enumeration, which is shown in Fig. 83, 

 is based on the assumption that the position of the stone-canal 

 and primary water-pore is the same in all classes. If that 

 assumption is incorrect, which it may very well be, j" the homolo- 

 gies which might be deduced from it fall to the ground. In 

 any case we desire to warn the reader against attaching too much 

 importance to the determination of homologies based solely 

 on this assumption. We have adopted the enumeration because 

 it conduces to clearness and enables us to impart a greater 

 precision to our descriptions and not because we think that it 

 is of any importance from the point of view of determining 

 homologies. 



The interradius in which the hydrocoel closes in the larva 

 presents remarkable variations, in relation to the position of 

 the madreporite, as the diagram indicates, and it is probable 

 that later researches will show an even greater variability in 

 this respect, the number of species hitherto examined from 

 this point of view being small. 



Our enumeration is identical with that of Loven excepting in its applica- 

 tion to Crinoids (see p. 272). The first-formed water-pore of Criiioids 

 and the anus are in the same interradius. As our enumeration is based 

 on the position of the water-pore, we assign the anus of Crinoids to inter- 

 radius II. Ill, and not to interradius I.V, as is done by Loven. 



One of the most important characteristics of the Echino- 

 dermata is the presence of calcareous plates in the dermis. 

 These may have various forms, from the isolated plates and 



* In Qphiuroids and Crinoids, though the number of radii is usually 

 five, the arms in some cases branch. 



f It is quite likely, as suggested by MacBride, that the interradius of 

 closure is the fixed point, the position of the madreporite being variable. 

 He points out to me that the stone-canal opens on the inner side of the 

 hydrocoel ring, and that the position of the opening might easily shift. 

 The madreporite being on the dorsal surface might also easily change its 

 interradius, as the anus undoubtedly does. 



