1863.] DR. J. S. BOWERBANK ON THE SPONGILLID^. 441 



brittle, and coarsely fibrous ; spicula linear and doubly pointed." 

 Dr. Johnston, in his ' History of British Sponges and Lithophytes ' 

 (published in 1842), adopts the two species established in Fleming's 

 work, but restores them to Lamarck's genus Sponffilla. 



Dr. Fleming was perfectly right in referring the British Spongillas 

 to the genus Halichondria as then constituted, as in the anatomical 

 structure of their skeletons they do not differ in any respect from a 

 very considerable number of British Sponges which were then in- 

 cluded in that genus, but which I have now found it necessary to 

 arrange separately in the genus Isodictya, with which genus, as 

 far as regards the peculiarities of the structure of the skeleton, 

 they are still identical ; but they differ from it materially in their 

 reproductive organs. In Isodictya the mode of reproduction is by 

 internal gemmulation, while in Sponyilla the same vital function is 

 always exercised through the medium of ovaria ; and in these organs 

 a peculiar structure and class of spicula prevail, which are never 

 found in the reproductive organs of any of the species of the marine 

 genus Isodictya. Their marked difference from that genus in so 

 important a function, and the striking and constant peculiarities of 

 the organs appropriated to that purpose, fully warrant their being 

 retained as a distinct genus under Lamarck's designation of Spon- 

 gilla, in accordance with the opinions expressed by that author, as 

 published in the second edition of his ' Histoire Naturelle des Ani- 

 maux sans Vertebres ' (vol. ii. p. 111). But the description of the 

 genus, as there given, does not appear to me to be sufficiently defi- 

 nite, and I have therefore endeavoured to amend it in the third part 

 of my paper on the "Anatomy and Physiology of the Spongiadce," 

 published in the second part of the ' Philosophical Transactions of 

 the Royal Society ' for 1862, p. 1115, as follows : — 



" Spongilla, Linnaeus, Lamarck, and Johnston. 

 Halichondria, Fleming. 



" Skeleton without fibre, composed of a symmetrical network of 

 spicula ; the primary lines of the skeleton passing from the base or 

 centre to the surface, and the secondary lines disposed at about right 

 angles to the primary ones. Reproductive organs ovaries, coria- 

 ceous, abundantly spiculous." 



Although Dr. Johnston adopted the two British species as de- 

 scribed by Dr. Fleming, he still retained doubts as to their being in 

 reality more than one ; and it was not until I had made careful mi- 

 croscopical examinations of the ovaria of each that their distinctive 

 specific characters were determined to my own satisfaction. 



If we partially dissolve these organs in hot nitric acid, we find the 

 spicula of the walls of the ovaries of S. flumatilis consisting of biro- 

 tulate forms, having their axes disposed at right angles to the sur- 

 face ; while the spicula of the ovaries of S. lacustris are simple and 

 elongate, and are disposed parallel to the surfaces of those organs, 

 thus affording occult but certain distinctive characters, without which, 

 from the great similarity in their habits and skeleton-structures. 



