STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 247 



collared cells, and pierced by an osculum and prosopyles, is 

 universally admitted to be the ancestral form of all calcareous 

 sponges. If we look for a more advanced type amongst the 

 Homocoela, from which the Heterocoela may possibly be de- 

 rived, the radiate section of the genus Leucosolenia at once 

 suggests itself. The type of this radiate section is Leucoso- 

 lenia tripodifera (1) ; and Bidder (21) is of opinion that 

 certain other species, notably Leucosolenia Lieberkuhnii, 

 should also be included therein. In L. tripodifera we find a 

 single wide central tube, with a terminal osculum, numerous 

 prosopyles, and a thin wall lined by collared cells — so far a 

 typical Ascon individual. From this tube, numerous radially 

 arranged branches are given ofi", which themselves branch copi- 

 ously, and terminate all at about the same level in blind, 

 rounded extremities, which touch one another, and thus form 

 an even surface to the whole sponge. Each of the radial 

 branches repeats, on a smaller scale, the structure of the 

 parent tube, and each is first formed as a hollow bud or out- 

 growth from the latter, the youngest buds lying at a little 

 distance below the osculum. The radial tubes occasionally 

 anastomose and inter-communicate, like the branches of a 

 reticulate Leucosolenia, but this appears to take place only 

 occasionally ; at any rate, it is not a characteristic feature. The 

 skeleton of this sponge consists of rather slender sagittal tri- 

 radiates and quadriradiates, and of the " tripod " spicules. 

 The latter are confined to the distal extremities of the radial 

 chambers. The sagittal radiates are arranged in a single layer 

 in the walls of the tubes, the quadriradiates with the apical ray 

 projecting into the gastral cavities both of the central tube 

 and its radial branches. As already stated, the spicules are 

 sagittal, and in the central tube the basal rays point towards 

 the base of the sponge; while Id the radial tubes the basal 

 rays point towards the blind distal ends, exactly as in the 

 articulate tubar skeleton of Sy conoid Heterocoela. 



Here, then, we have a form which makes a very near 

 approach, both in canal system and skeleton, to the Sycetta 

 type amongst the Heterocoela, and it is from some such 



