222 ANATOMY AND PHYSIOLOGY 



organic specific characters, many of which are exceedingly 

 prolific in materials for descriptive purposes. A long 

 familiarity with them has assured me of their value, and of 

 their constancy in each species. However protean the 

 form and colour may be, the organic structures can always 

 be recognised with certainty, provided the specimen under 

 examination has been dried in the condition in which it 

 has been taken from the sea. To the organic characters 

 may be added the less definite and valuable ones of form 

 and mode of growth, which, although less to be depended 

 on than the organic ones, are frequently of service in con- 

 junction with them, as leading and suggestive in the first 

 stage of investigation. 



A dependence on the specific characters to be derived 

 from form alone inevitably leads to erroneous conclusions. 

 Thus from trusting too implicitly to it in the descriptions 

 of his species, Dr. Johnston, in his ' History of British 

 Sponges/ has made two species out of one in the case of 

 Dysidea fragiliSi the thin coating form of this sponge being 

 also described as Halichondria areolata. Halichondria 

 incrustans has also been described a second time as Hal. 

 sabiirrata. An elongated form of Halichondria ficus has 

 also been again described as Hal. mrgultosa. The type- 

 specimen of Halichondria sevosa, Johnston, in the British 

 Museum proves to be merely a thin coating variety of 

 Halichondria panicea ; and the type-specimen of Montagu's 

 Spongia digitata in the possession of Professor Grant, Hali- 

 chondria cervicornis, Johnston, on being microscopically 

 examined, proved not to be a sponge but an alga. 

 Numerous other instances of error arising from a depend- 

 dence on form alone as a specific character might be cited, 

 but those I have given above are sufficient to prove the 

 ineligibility of so mutable a character unaccompanied by 

 organic structure. 



Nearly the whole of this extensive series of specific cha- 

 racters have hitherto not been applied in the descriptions 

 of the SpongiadaB, excepting in my own manuscripts. This 

 omission has occurred, not from any doubt of their value, 

 but simply because they were unknown to naturalists. It 



