Siliceous Spicules of Sponges. 215 



spicules ; but it is much more difficult witli those sponges where 

 two or even more types of spicules are found in connexion 

 with the usual simple ones (in fact, having the character- 

 istics of more than one group) — as, for example, in the coral- 

 loid sponges, where we have the sexradiate spicules well 

 developed in conjunction with the more or less discoidal spi- 

 cules, which have a resemblance to the trifurcate spicules, 

 typical of another group. 



In the same way it is not uncommon to find the spicular 

 spherules (which are spherical masses of spicules so character- 

 istic of Geodia) in conjunction with quinqueradiate spicules 

 (which are characteristic of a different group) ; and hamate 

 spicules, which are frequently only found in conjunction with 

 simple unbranched spicules, are also found with quinque- 

 radiate spicules, and, I believe, rarely with sexradiate spicules. 



These facts only go to prove that there are in reality no 

 very broadly marked groups, especially of a secondary degree, 

 in Nature, and that there are a number of intermediate forms 

 which are to be arranged along with the other genera of the 

 group to which they bear the greatest resemblance in other 

 respects. 



However, there is one evil particularly to guard against, 

 especially with regard to hamate spicules — to make sure that 

 they really belong to the sponge in which they are found, 

 and are not accidentally intermixed with it from the sea and 

 other external sources. 



When the spicules of a species are figured, it is very de- 

 sirable that the comparative size of the different forms of spi- 

 cules should be carefully preserved, and, if any spicule is figured 

 of a larger size than the others, that the difference of its scale 

 should be stated. The want of attention to this point greatly 

 detracts from the value of Dr. Schmidt's figures. I could 

 not believe that the slides of the spicules of the different species 

 that he had sent me were correctly named, when I com- 

 pared them with his figures, until I observed that he had paid 

 no attention to this circumstance, and that in a figure that 

 appeared to be taken from one group of spicules in a slide, the 

 smaller spicules were represented on a much greater scale, and 

 •therefore appeared to be larger than the greater ones. 



One must not confound the siliceous spicules of sponges 

 with the horny stars described and figured by Mr. Carter as 

 occurring in Aplysina corneostellata from the coast of Spain 

 (Ann. & Mag. Nat. Hist. 1872, x. p. 105, t. vii. f. 1), which 

 is most probably the sponge that the Germans have described 

 and written so much about under the name of Darwinella ; 

 but these horny- fibred, six-rayed stars have no relation to 



