Classification of the Asconidge. 361 



Nevertheless it does not seem to me improbable that these 

 forms should exist, and their relations to other Ascons would 

 certainly prove of great interest. They probably occupy 

 somewhat the same relation to the species of the genus Leuco- 

 solenia that the Ascetta forms of the genus Clathrina occupy 

 to the remaining species : but in one respect they are probably 

 much more primitive than any other Ascons ; for if the mon- 

 axon spicules composing the skeleton are, like those of the 

 young variabilis^ secreted each by a cell of the flat external 

 epithelium, then, since the layer of triradiate systems is 

 absent, there should be no cells between the flat epithelium 

 and the gastral collar-cell layer, except the wandering and 

 genital cells — in other words, the so-called mesoderm might 

 be expected to be practically entirely absent, and, if so, the 

 two species of Ascyssa would be more primitive in structure 

 than any other known Ascons. 



To predict is always dangerous ; but I venture to believe 

 that these Ascyssa species, if they exist, will prove on inves- 

 tigation to be on the Leucosolenia stem, so to speak, and will 

 be found to have collar-cells with the nucleus in the upper 

 portion and an amphiblastula larva, or, at least, a larva more 

 resembling the amphiblastula than the parenchymella. 

 Hackel's figure of Ascyssa acufera^ it may be noticed, shows 

 a typical arborescent Leucosolenia-\Sk& colony *. 



Ascandra falcata is an interesting form which has often 

 been regarded as intermediate to a great extent between 

 Ascons and Sycons. But this does not seem to be true, at 

 least if we take as typical of the Sycons such a form as Sycon 

 rajphanus ; for while Ascandra falcata is shown, by all its 

 characters^ to be distinctly on the Clathrina stem, Sycon 

 raphanus^ on the other hand, is plainly allied to the genus 

 Leucosolenia ; it is distinguished from the latter only by its 

 form and mode of growth and by the restriction of the collar- 

 cells to the radial tubes ; but in all other characters it agrees 

 with it, namely in the sagittal spicules, the collar-cells with 

 terminal nucleus, the amphiblastula larva, and the transitory 

 Ascyssa stage in the development. Bidder f has well pointed 

 out the importance of the arrangement of the spicules in the 

 radial diverticula of the oscular tubes of Leucosolenia Lieber- 

 kuhnii, an arrangement which shows plainly that in these 

 diverticula the current is at first centripetal, so that they are 

 strictly comparable, as long as they have not exceeded a 

 certain length, to the radial tubes of a Sycon. The number 



* Hackel, ' Die Kalkschwaiunie/ Taf. vii. fig. 4. 



t " Review of Dendy's ' Monooraph of the Victoriau Sponges,' "Q, uart. 

 Journ. Micr. Sci. n. s. xxxii. (1891) p. 627. 



