BRITISH SPONGIAD^. 341 



metnbrane, and th© iritier one performing the sarae oflSce 

 for the mtertoal 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 Histofy regarding the number of 

 our native species of BpongiUa. Ray (Syh. Stirp. 30) 

 notiices two: species under the designatioA' of " Spmgia ra- 

 mosa fluviOitilu" and Spongia flnviaiilis ramosa frapilis." 

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

 History' (vol. ii. p. 420, published in 1803), 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 j 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 ' Histoi-y of British Animals' (p. 524, 

 published in 1828), describes two species under the generic 

 designation of Halichondria: — " H. fluviatilis. Soft, 

 brittle, and slenderly fibrous when d!ry ; 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), adk)pts the two speties 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 Halichondrict, as then constituted, 

 as in the anatomical structure of their skeletons they do 

 not difier 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 an?ange sepa- 

 rately in the genus Isodictya, and with which' genus, as fai* 

 as'i'egards the peculiarities of the structure of the skeleton, 

 they are stiU identical ; but they dififer from it materially 

 in their reproductivie organs. In Isodictya, the mode of 



