BRITISH SPONGIADjE. 341 



membrane, and the inner one performing the same office 

 for the internal membrane ; but they are so completely 

 covered by the respective membranes, that without the 

 application of nitric acid they would be extremely likely to 

 escape observation. 



Much uncertainty appears to have existed among' our 

 early writers on Natural History regarding the number of 

 our native species of Spongilla. Ray (Syn. Stirp. 30) 

 notices two species under the designation of " Spongia ra- 

 mosa fluviatUis" and Spongia fluviatUis ramosa fragilis." 

 Charles Stewart, of Edinburgh, in his ' Elements of Natural 

 History' (vol. ii. p. 420, published in 1802), describes one 

 species in the following terms : " Spongia lacustris. Creep- 

 ing on other bodies and taking their figure ; brittle, with 

 erect, round, obtuse branches. Inhabits England, Sweden, 

 &c. This species is found in lakes and rivers; it has a 

 strong peculiar smell ; when young, flat ; when old, putting 

 forth branches. In autumn it contains little globules, like 

 seeds, which explode when put into the flame of a candle." 



Fleming, in his ' History of British Animals' (p. 524, 

 published in 1828), describes two species under the generic 

 designation of Halichondria : " Jf. fluviatUis. Soft, 

 brittle, and slenderly fibrous when dry ; spicula linear and 

 doubly pointed. H. lacustris. Hard, brittle, and coarsely 

 fibrous ; spicula linear and doubly pointed." Dr. John- 

 ston, 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 

 Spongilla. 



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 included in that genus, 

 but which I have now found it necessary to arrange sepa- 

 rately in the genus Isodictga, and 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 Isodictga, the mode of 



