STUDIES ON THK COMPARATIVE ANATOMY OF SPONGES. 189 



rays being all alike, with the angles between them equal. (2) 

 Sagittal, when two of the facial rays or two of the angles form 

 a pair dijQfering in some respects from the remaining ray or 

 angle. In this case the paired rays are termed oral, the odd 

 ray is termed basal, and the odd angle between the oral rays 

 is also termed oral. (3) Irregular, when the three facial rays 

 conform to neither of the above plans. 



(c) Oxeote, 

 or uniaxial spicules (oxea), in which there is only a single 

 axis. These spicules vary greatly in details of shape, from 

 symmetrically fusiform, with two sharply pointed ends, to nail- 

 shaped, with one end blunt and swollen. 



As in the Homocoela, so in the Heterocoela, the mere 

 presence or absence of one or other of these three types of 

 spicule is of very slight — in my opinion, of not more than 

 specific — value for purposes of classification. With the arrange- 

 ment of the spicules I believe the case to be totally different, 

 and I find, in the strncture of the skeleton as a whole, charac- 

 ters of family value to the systematist. 



The Arrangement of the Spicules. — Here, as in the 

 case of the canal system, we shall find it most convenient to 

 take the various genera one by one and deal with them 

 separately. We shall find that, up to the Grantia stage, the 

 arrangement of the skeleton follows, and appears to be to a 

 large extent controlled by, that of the canal system. But on 

 reaching the Grantia stage the development of a strong 

 dermal cortex introduces new possibilities with regard to the 

 skeleton, which commences to vary independently of the canal 

 system, and branches off along three lines corresponding to the 

 families Grantidse, Heteropidse, and Amphoriscidse. The genus 

 Leucascus appears to occupy an isolated position. 



Leucascus (fig. 1). 

 In this genus (fig. 1) the arrangement of the skeleton is 

 extremely simple, and exhibits no trace of that radial sym- 

 metry which is so characteristic of the Heterocoela. It 



