STUDIES ON THE COMPAEATIVE ANATOMY OF SPONGES. 249 



Leucosolenia tripodifera, and that the radiate Heterocoele 

 type was developed from this by replacement of the collared 

 cells of the central gastral cavity by a flattened epithelium and 

 specialisation of the skeleton, a modification which might 

 readily give rise to the primitive Heterocoele genus Sycetta. 

 The manner in which I believe the majority of the remaining 

 genera of Heterocoela to have been derived from a Sycetta- 

 like ancestor, has perhaps been already sufficiently indicated 

 in dealing with the anatomy and classification of the group. 

 The genus Leucascus, however, does not appear ever to have 

 passed through a Sycetta stage, for while the greatly 

 elongated flagellated chambers may indicate by their arrange- 

 ment (at any rate in some specimens) a certain degree of radial 

 symmetry, the skeleton exhibits none whatever; it is quite 

 as irregular as that of a reticulate Homoccele, or as that 

 of the most modified Leucandra. Considering the nature 

 of the canal system, however, I believe that the irregularity 

 of the skeleton in Leucascus is a primitive condition, and 

 not, as in Leucandra, a secondary one. Possibly Leucascus 

 may be either derived directly from a reticulate Homoccele 

 ancestor, by the formation of a true dermal membrane and 

 dermal pores, or from a very low type of radiate Homoccele, 

 in which the skeleton had not yet acquired any radial symmetry. 

 I am inclined to adopt the latter view. 



The opinions as to the origin and inter-relationships of the 

 Calcarea Heterocoela which I have endeavoured to justify 

 in the foregoing pages, may be conveniently summarised in 

 the accompanying diagram, which naturally takes the form 

 of a genealogical tree. 



