AND PHYSIOLOGY OF THE SPONGIAD^. 
1129 
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 descrip- 
tions of his species. Dr. Johnston, in his ‘ History of British Sponges,’ has made two 
species out of one in the case of Dysidea fragilis, the thin-coating form of this sponge 
beinfir also described as Halichondria areolata. Halichondria incrustans has also been 
O 
described a second time as H. saburrata. An elongated form of Halichondria ficus 
has also been again described as H. virgidtosa. 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 Geant, Halicho7idria cervicornis, Johnston, on being microsco- 
pically examined, proved not to be a sponge, but an alga. Numerous other instances of 
error arising from a dependence 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 characters have hitherto not been 
applied in the descriptions of the Spongiadse, 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 now remains to be proved how they may be rendered 
available in future descriptions of those animals. I cannot, perhaps, better attain 
this end than by detailmg the order and mode of employing them in the description 
of species contained in my own Manuscript History of the British Sponges. The 
following is the order in which these characters have been taken for examination and 
description : — 
1. Form. 2. Mode of Growth. 3. Surface. 4. Oscula. 5. Pores. 6. Dermis, and 
Dermal Membrane and its Spicula. 7. Skeleton and its Spicula. 8. Connecting Spi- 
cula. 9. Defensive Spicula — external, internal. 10. Spicula of the Membranes — 
tension spicula, retentive spicula. 11. Sarcode and its Spicula. 12. Ovaria and Gem- 
mules, and their Spicula. 
Colour. 
Habitat. 
Condition when examined. 
This order of description, or any other that the student may prefer, should always be 
adhered to, and no part of the specimen under examination that is present, and which 
affords specific characters, should be omitted in the description ; so that, when no men- 
tion is made of particular organs or classes of spicula, it may be presumed that they are 
not present in the sponge in course of description. A certain portion of these charac- 
ters are always available. Thus the skeleton, incurrent canals or cells, the sarcodous 
system, the dermal and interstitial membranes, the pores, and the oscula are always 
present, while the excurrent canals or the cloaca are occasionally absent. The inter- 
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