56 J. G. WALLER ON VARIATION 



S. Parfitti alone, applied to a slight variation. But if we retained 

 "fluviatilis " we should naturally infer, whatever additions we 

 gave, that it recorded a variation only from the known type ; and it 

 may be that these observations might be extended even to some of 

 the exotic species, for the multiplication of terms is in itself an evil, 

 threatening to make us little better than walking catalogues. 



Now the first of the Thames S. fluviatilis to which I shall call 

 your attention is that found in Tecldington Lock, because this comes 

 as near as possible to that from the Exe, to which I have referred. 

 In external appearance it partakes of the foliate or lobate form of 

 one of the exam]3les engraved by Dr. Bowerbank, Vol. iii, p. 295, fig. 7, 

 but this is very common to the genus. The spicules of the skeleton 

 are chiefly of the ordinary smooth type, acerate ; but among these 

 is a variation almost cylindrical, the terminations being acutely 

 pointed, making the whole not unlike a lead pencil, sharpened at 

 each end. It is not, however, common only to this, as it occurs, 

 though in a less degree, in most of the species. The rest of the 

 spicules, perhaps about one-third, are spinous, like that of the Exe, 

 sometimes slightly so, but some have the spines well developed. It 

 is an interesting fact to which I must syjecially direct your attention 

 that the growing fibres of the apices of the sponge have all the 

 spicules smooth, and it is only as we proceed downward towards the 

 base and older parts that the spinous character is produced. This 

 in itself gives us a clear hint that the latter belong to a development 

 which other examples, soon to be described, have carried still 

 further. An abnormal form, which has often been found in the 

 species, I have only found here. It is a spicule, somewhat sym- 

 metrical, having a central inflation, often two, and even three, and 

 found dispersed throughout the tissues. I record it merely as a 

 form, in which the Spongilla sports, not as having any specific value. 

 As regards the birotulate spicules they accord with that from the 

 Exe, being exactly the same size, viz., ^Vo °f an inch, hut I have 

 not found the shaft so frequently spined. (PI. V., fig. 1.) 



Leaving Tecldington Lock, we will proceed to that of Molesey, 

 near Hampton Court, where this Spongilla grows in luxuriant 

 abundance. Nowhere else have I seen such fine specimens. Here 

 I have never seen the foliate condition, for the maturely developed 

 sponge has a generally even surface, undulating slightly, but showing 

 no differences from that which is typical. It is when its structure 

 is examined under the microscope that its interest begins. Rising 



