576 E. A. M[NCHIN. 



usually much smaller^ and may be reduced to a mere nodule, 

 or may even be quite absent. In the latter case, which is of 

 very frequent occurrence, the spicule becomes simply a curved 

 monaxon. Lendenfeld has figured what I take to be some 

 spicules of this class (1891, pi. viii, figs. 3, c, d, e), but I cannot 

 say that his figures represent the forms of these spicules as I 

 am familiar with them. Bidder terms them tripods (1891, 

 p. 627). I regard these spicules, which show every variation 

 between two extreme forms, the one triradiate, the other 

 secondarily monaxon, as the homologues of the monaxons of 

 contorta, and like them they become crystalline much earlier 

 than the regular triradiates, and can be picked out in the 

 embryo by the brilliance with which they light up between 

 crossed prisms. While the spicules become crystalline at a 

 relatively late period in contorta and cerebrum, in falcata 

 they do so much earlier, so far as my observations go ; the 

 rays are separate and non-crystalline till they have a length of 

 about 3 fi, then they unite and show crystalline properties. 

 In reticulum, finally, I have not been able to find any 

 spicules, however small (2 fx) which have the rays either 

 separate or non-crystalline. The smallest visible spicules 

 glitter like stars between the prisms. 



The comparison of different species seems to show that the 

 triradiate spicules are the product of a long course of evolution 

 in a certain direction, the furthest point in which has been 

 attained by reticulum. If the view which I have taken of 

 the phylogeny of the spicules be the true one, — if, that is to 

 say, they arose primitively from fusion of three or four separate 

 monaxon spicules, — it seems to me highly probable that 

 the ancestral triradiate or quadnradiate spicules, when they 

 first appeared, would not have behaved optically as single 

 crystals ; and that the fact that they do so now is a secondary 

 character, the result of their ontogeny. Instead of now being 

 formed as three (or four) distinct spicules which fuse together 

 when full grown, the relations of the rays are in a certain 

 sense predestined before they are secreted. They arise in 

 close contact, and fuse when still excessively minute ; hence 



