58 J. G. WALLER ON VARIATION 



spinous one, and become practically obsolete. Place the two 

 extremes side by side and the divergence is great ; follow the 

 development through the Teddington and Molesey examples, and 

 the connection is clear enough. Nor are the localities cited the sole 

 places on the Thames in which these varieties are found. At Cleeve, 

 in Oxfordshire, is a parallel to that of Molesey ; whilst at Goring, 

 in the same county, we get that of Surbiton. But, as yet, the 

 Thames has not furnished me with a single example coinciding with 

 Dr. Bowerbank's type found in the West Country Timber Dock, and 

 that locality has failed to give me a specimen. But the river, 

 nevertheless, has given one with the smooth spicule only, but it 

 provokes us, by not adhering to the type ; so even now we do not 

 get rid of variation. This is a parasitic form, found at Henley ; 

 not, however, in the full flow of the river, but in a small side creek ; 

 a difference of condition which should always be noted, although at 

 present we may not be able to draw conclusions from it. Here we 

 find the dermal membrane completely matted with a dense mass of 

 spicules, to a degree not often seen in the Spongiadas at all — still 

 less in a sponge in which its absence is declared to be a typical 

 character. Noting that the shaft of the birotule is longer than 

 those described, and I need not further detain you on this form, 

 which concludes our Thames Spongillas. 



I have not, however, done with varieties ; for, having 

 spoken of the parasitic form at Henley, I am now about 

 to introduce to you another, which, in many respects, exceeds 

 the rest in interest. This was discovered in a large pond, 

 or lake, of two acres in extent, at Ditchleys, in Essex, growing upon 

 the stems of aquatic plants at the bottom. The structure of a 

 sponge will often depend on the base upon which it grows — certainly 

 this is the case with the class to which the Spongilla naturally 

 belongs. So one having a broad base will have its network regular 

 and symmetrical ; whilst another, completely like it in other respects, 

 but springing from a narrow axis, will appear very irregular. This 

 is particularly seen in the example under consideration, its surface 

 is flexuous and plumose. But, here, as in all other cases, the in- 

 terest consists in a more intimate examination. In this, the spicules 

 are all smooth, and, in all the parasitic forms, this seems to be 

 the case ; but here, not only is the dermal membrane thickly fur- 

 nished with spicules, but the interstitual membranes also have them ; 

 they are of a more delicate character than the skeleton, though 



