382 NOTES ON PLYMOUTH SPONGES. 



littoral species such a detennination would in many cases be quite 

 meaningless. 



The cylindrical Sycon compressuni, referred to in the previous note, 

 may be considered a metamp of the flat Sycon conqyrcssum. In this 

 case the flat form may be considered normal, since according to present 

 knowledge it is a thousand times more frequent. Yosmaer (Mitt, aus 

 d. Zool. Stat. z. Neapel, vol. v. part 3) has put forward the view that 

 Lencandra aspera is extraordinarily diverse according to its habitat, 

 and my own work has caused me to take the same view of Sycon 

 raphanus (cf. the papers cited in previous note) ; these are cases of 

 mutual and probably continuous metain})y, where a normal form is 

 diflicult to select. 



The skin of H. panicca, with its net of spicules, is well represented 

 in Bowerbank's figure {Monogrcqjh, vol. i. Fig. 505). It is coherent and 

 easily detachable, and, as Kidley and Dendy remark {Challenger Mon., 

 p. 15), Schmidt's definition of his genus Pcllina is completely fulfilled 

 by this species. Topsent, however, has since {Bdformc dans let Classifi- 

 cation des Hcdichondrina) revived the genus Pellina, with a Eenieran 

 instead of a Halichondrian skeleton. 



The skin is considerably thicker and more spicular in the Plymouth 

 than in the Exmouth specimen, being conspicuous in spirit as a white 

 veil over the sponge, showing as a hard white line when the sponge is 

 cut. It is tempting to connect this denser spicule-sheathing of the 

 tidal sponge with the restraint of evaporation and protection of soft 

 tissues against brine and I'ain-water, as suggested in regard to the 

 club-spicules of S. compressuni ; but it must be remembered also that 

 far denser crusts are well known in other siliceous genera which are 

 not tidal. 



Bowerbank's Fig. 300 does little justice to the subdermal space. 

 This is not a series of spherical lacuna3 in the tangle of spicules ; the 

 spicules in the ectosome of If. p)cmicca are as definitely arranged as in 

 an Axinella. The skin is only connected with the body by spicule- 

 fibres, which traverse the subdermal space like the columns in a 

 Norman crypt, expanding above to support the tangential spicules of 

 the skin, and below to root in the body. 



!! 



