190 AETHUE DENDY. 



resembles^ on the other hand, the arrangement of the skeleton 

 in such simple Homocople sponges as Leucosolenia proto- 

 genesj excepting that there is a true dermal skeleton in the 

 pore-bearing dermal membrane. In both species of Leu- 

 cascus (4) the skeleton consists of small, regular triradiates, 

 irregularly scattered in the walls of the elongated chambers 

 and exhalant canals, and in the dermal membrane. In Leu- 

 cascus simplex these are the only spicules present, but in 

 L. clavatus we find in addition some large, club-shaped, 

 uniaxial or oxeote spicules, partly projecting from the dermal 

 surface. 



Sycetta. 



Here, in accordance with the arrangement of the canal 

 system, we can distinguish between two main parts of the 

 skeleton, — (1) the gastral skeleton, supporting the wall of the 

 gastral cavity; and (2) the tubar skeleton, supporting the walls 

 of the radial chambers. The gastral skeleton consists of tri- 

 radiate or quadriradiate spicules, whose three facial rays lie in 

 the thickness of the wall of the gastral cavity, while the apical 

 ray, if one happens to be developed, projects into the cavity. 

 These spicules may be sagittal, as in Sycetta (Sycaltis) coni- 

 fera (5), and then the basal ray is found to be directed down- 

 wards, away from the osculum and towards the base of the 

 sponge, a position which is so constant in this and other genera 

 as to have given rise to the term "basal ray.^^ The tubar skeleton 

 consists exclusively of triradiate spicules, which lie in the thick- 

 ness of the chamber walls, and which always have the basal ray 

 directed along the length of the chamber and away from the 

 gastral cavity. Hence the oral or paired rays are spread out in 

 a direction at right angles to the length of the chamber, and as 

 several spicules generally lie at the same level, the tubar 

 skeleton forms a series of more or less definite joints or rings, 

 and is hence said to be articulate. This articulate arrange- 

 ment, however, which prevails in most genera with a Syconoid 

 type of canal system, is usually very irregular. 



