188 ARTHUE DENDY. 



species of Leucandra and Leucilla, and presumably also in 

 Lelapia and Leucyssa. 



No one of these five stages is very sharply marked off from the 

 stage below it, and the five appear to me to indicate a process 

 of evolution which has actually taken place. Von Lendenfeld 

 (10) attributes to the Leuconoid type of canal system an inde- 

 pendent origin from the Homocoele typCj through his very pro- 

 blematical '"^Leucopsidse/' without passing through a radiate 

 Syconoid stage at all, although he admits the derivation of 

 the Sylleibid type from the latter. With this view I cannot 

 at all agree. Considering the canal system alone, we have 

 cogent reasons for opposing it, and when we come to discuss 

 the skeleton we shall find others. 



III. The Skeleton of the Calcarea Heteroccela. 



The Spicules. — So much has been written about the 

 spicules of calcareous sponges, and their variations in minute 

 details of shape are of so little interest from a morphological 

 point of view, that I propose to say very little about them in 

 this place, and only to recapitulate those facts which it is 

 necessary to consider in discussing the arrangement of the 

 skeleton. 



In calcareous sponges, whether Homocoele or Heterocoele, 

 three principal types of spicules are met with : 



(ft) Triradiate, 

 with three rays diverging from a common centre and lying 

 typically in one plane, though frequently curved more or less 

 out of that plane. 



(b) Quadriradiate, 

 resembling the triradiate but with an additional ray, known as 

 the apical ray, coming off from the centre in a plane at right 

 angles to the plane of the other three (facial) rays. 



In both triradiate and quadriradiate spicules we can dis- 

 tinguish three chief varieties : — (1) Regular, the three facial 



