228 AETHUR DENDY. 



Family 1. — Leucascid^, Dendy (4). 

 Diagnosis. — Flagellated chambers very long and narrow, 

 copiously branched; communicating at their proximal ends 

 with exhalant canals, which converge towards the oscula; 

 their blind distal ends covered over by a dermal membrane 

 pierced by true dermal pores, which lead into the irregular 

 spaces between the chambers. Skeleton consisting princi- 

 pally of small radiates, irregularly scattered in the walls of 

 the chambers and exhalant canals, and in the dermal mem- 

 brane. 



Genus 1. — Leucascus, Dendy (4), fig. 1. 



Diagnosis. — The same as that of the family. 



E-emarks. — Leucascus, the sole genus of the family, 

 appears to occupy a very isolated position amongst the 

 Heterocoela, both as regards skeleton and canal system. 

 Its skeleton, as already pointed out, has not yet attained to 

 the typical radiate condition of the Heterocoela, but retains 

 the irregular scattered character of the reticulate Homocoela. 

 The genus might, indeed, be easily confounded with the 

 reticulate section of the genus Leucosolenia (1), but differs 

 in several very important particulars; viz. (a) The possession 

 of a distinct pore-bearing dermal membrane, which is found 

 in no Homocoele sponge with which I am acquainted, and is 

 not even developed amongst the Heterocoela until we reach 

 the most advanced Sycon condition. The pseudoderm 

 of many reticulate Homocoela (1) is by no means an homo- 

 logous structure, {b) The absence of collared cells from the 

 exhalant canals, which there is no reason for believing to be of 

 a '^pseudogastral '^ nature, as in my type D of the reticulate 

 Homocoela. (c) The flagellated chambers, although, owing 

 to the massive form of the sponge and the absence of a single 

 central gastral cavity, they cannot be truly radial, nevertheless 

 show a marked tendency to become so. 



The genus may, perhaps, be best regarded as derived inde- 

 pendently from a lowly organised type of radiate Homocoela. 



