62 Mr. G. P. Bidder. 



sectional area) is the manifestation of a pressure in the flagellate 

 chambers (of broad aggregate sectional area); which pressure is 

 maintained by the moving flagella, and resisted by a normal tension 

 in the walls of the chambers or of the sponge.* 



I therefore regard the contraction recorded by Minchin in C. 

 clathrus (and undoubtedly occurring in unfavourable conditions, not 

 only in this, but in many other sponges) as the passive result of per- 

 manent tensions in the sponge-walls, which are no longer counteracted 

 by the comparatively inactive flagella. f It is with the advantage of 

 preventing such contraction being a consequence of inactivity that a 

 skeleton has been evolved in most sponges : whatever the substance 

 of which ib is composed, this skeleton tends to -take in surfaces a tri- 

 radiate [more rarely sexradiate] arrangement. 



[If a uniform elastic membrane be extended by a star of rigid 

 rods, the area extended is greatest in proportion to the material in- 

 volved, when the star is equiangular and has either four or five rays; 

 though there is comparatively little advantage for any number from 

 three to six. Where, however, the rigid rods unite in a network, 

 the triradiate (i.e., hexagonal) arrangement is the most economical 

 possible, and the triangular (or sexradiate) the most rigid. The 

 square superficial network is less unstable but less economical than 

 the hexagonal ; it would seem rarely to occur except where as in 

 the Hexactinellida and in Reniera a definite cubic mesh has been 

 developed to support a frothy tissue of three dimensions (c/. Schulze 

 (11) ).July 23, 1898.] 



Minchin, after a most interesting account of the ontogeny of 

 triradiate and quadriradiate spiculesj in reticulate Ascons, writes (21, 

 p. 549) : " Crystallisation cannot now be taken as an adequate 

 explanation of the external form of the spicules . . . It is . . . 

 clearly impossible that the triradiate system should owe its form, 



* Cf. (18), p. 29. I still hope to write more in detail on this subject. 



f The flask-like form of the ectocytes I still hold to have been " developed to 

 expose tlie greatest possible surface to the medium from which the excreted " (or 

 secreted) "substance is derived," (16), p. 480. The evidence appears to me still 

 iu favour of these cells being excretory ; but whatever their secretion, it is im- 

 portant to the sponge, and did they not take this form on contraction of the sponge, 

 their surface in contact with the parenchymal jelly would be di>advantageously 

 reduced. I regard as an extreme case of this the spongoblasts [considering the 

 fibre to be an invaginated tube of cuticle contracted by its own elasticity until it 

 has become a solid rod.] 



J I may mention that Sycon compressum has hair-spicules about -^p. in thickness 

 which are each formed by two cells, as Minchin describes for a ray of a triradiate 

 in Clathrina ; and the young giant club-spicules also show two formative cells. 

 For such pairs of cells I suggest the name " adelphidia " ; they were described 

 by Lendenfeld (8) and Stewart (10) as sense-cells. 



The "triradiate system" is a word used by Minchin to denominate the three 

 rays of the triradiate spicule whether or not an adventitious fourth ray be added 



