62 J. G. WALLER ON SPONGILLA FLUVIATILIS. 



engaged in every department of life. It is therefore the more neces- 

 sary that obstacles to the study of a particular branch should not be 

 raised up by the creation of unmeaning names and complicated 

 terminologies. 



Description of Plate V. 



The following figures of the spicules from the several specimens of the 

 Spongilla Fluviatilis all drawn to the same scale, exceptiug the birotulate 

 forms (40 diam.), for the sake of easy comparison, illustrate the argument 

 of variation as against separation into species. Fig. 1 is a group from 

 the Teddington Lock example. Fig. 2, Molesey. Fig. 3, Surbiton. Fig. 4 

 is a group from that of Ditchleys, the finer and more slender spicules being 

 from the membranes. Fig. 5, from Bolton Mills Pool, Canterbury, gives 

 the single form of acerate spicule, which characterises the species, 

 and is remarkable for its large size, exceeding that of Sp. Meyeni, 

 which is given in fig. 6, and which but slightly exceeds that of 

 the longest from Ditchleys. The variation from the simple acerate to 

 the fusiform, is here also shown, as well as that of size, which differs in 

 every example ; those of Molesey being the smallest, the spinous one from 

 Teddington excepted. As to the birotulate spicules, they are drawn simply 

 as they exhibit themselves under the same magnification (440 diam) 

 and plane, as they vary much in appearance according to focussing. 

 But the general character is obviously the same throughout, and if 

 they differ from those published hitherto, as regards the flat face of 

 the rotule, it will be a question as to how far imagination has not assisted 

 in a rather difficult task, and led draughtsmen into seeing too much. The 

 dentate edges differ in plane, and consequently differ according as they are 

 focussed. The figures given are such as the several examples have exhibited 

 when the front of the rotule is shown ; and between them all, including 

 even Spongilla JJeyeni, there is no difference except in size. In figs. 1, 

 2, 3, the size is precisely the same, and agrees with Sp. Parfitti. The 

 irregularly dentate character generally prevails, but the deep, neat, and 

 more regular dentation given to that of Teddington, is but an 

 instance due to focussing, and can be seen in the two following 

 examples. The large size of the birotule of Sp. Meyeni, in its 

 long diameter, exceeds all others, but it fluctuates into smaller forms, 

 some of which are less than that of Bolton Mills Pool, whilst 

 the diameter of the rotule is nearly of the same size, and the birotule 

 of Ditchleys stands as intermediate between the less and the larger, 

 and is paralleled by that of Henley. The dentation is sometimes strongly 

 pronounced, and in others feebly out of examples from the same sponge, 

 and varies much in detail, preserving, however, the same general charac- 

 teristics. Fig. 7 is the gemmule (?) or ovisac found in the Ditchley and 

 Hertford Spongilla (125 diam.) 



