214 ANATOMY AND PHYSIOLOGY 



different, as to render them almost as valuable as if they 

 varied from each other in form. Wherever therefore spicula 

 form a component part of the skeleton, they become a leading 

 character in the discrimination of species. But it is not only 

 those of the skeleton that are thus available, as in different 

 sponges they vary in shape and size in each separate organ 

 belonging to the animal ; and in some cases we find as 

 many as five or six distinct descriptions of spicula, each of 

 which affords an invariable and excellent character. Thus, in 

 the descriptions of sponges, it is not only the forms and relative 

 proportions of the skeleton spicula which have to be taken 

 into consideration, but those also of the dermal and inter- 

 stitial membranes (the external and internal defensive ones), 

 those of the sarcode, and of the ovaries and gemmules. 

 Those of the latter three organs named frequently afford the 

 most determinative characters. Thus in the genus Sjjo?i- 

 yilla but one form of spiculum, the acerate, prevails in the 

 skeletons of nearly all the known species ; but the minute 

 and beautiful spicula of the ovaria varies in form and size in 

 each species in a perfectly unmistakeable manner, so that 

 if the organs of reproduction be present, which is most 

 frequently the case, the species may be readily recognised 

 from their spicula only. But in other cases, and even in 

 the same genus in the absence of the ovaria, the differences 

 between two nearly allied species are equally well deter- 

 mined by the spicula of the dermal and interstitial mem- 

 branes. Thus in our two species of British Sjjongilla, S. 

 ftuviatilis has no tension spicula different from those of the 

 skeleton, while in 8. lacustris we find the fusiformi- acerate 

 entirely-spined spiculum, represented by Fig. 90, Plate IV, 

 in abundance. So likewise in two species of Tethea, T. 

 cranium from Shetland, and T. simillima, Bowerbank, MS., 

 from the Antarctic regions, the only well- determined 

 difference that exists is, that the sarcode of the former is 

 profusely furnished with exceedingly minute sigmoid 

 spicula, while that of the latter is entirely destitute of them. 

 It will therefore be seen that these exceedingly minute 

 organs frequently afford the most valuable and certain 

 means of discriminating species. But although so minute, 



