OF THE SPONGIAD^E. 3 



Halisarca, and these nine genera are all that are contained 

 in Dr. Johnston's ' History of British Sponges.' 



Both of these arrangements are very insufficient, and 

 that of Lamarck completely ineffectual, inasmuch as there 

 is no class of animals in which the form varies to so great 

 an extent, according to the difference of locality or other 

 circumstances; and also even when there is a striking 

 normal form, it is rarely thoroughly developed until the 

 animal has reached its full maturity. According to the 

 practice of Lamarck, even under the most favorable cir- 

 cumstances there are frequently recurring difficulties in the 

 determination of the species by this method, as the same 

 forms are found to be common to a great number of 

 sponges, the internal organization of which are widely 

 different to each other. Prom these causes it is, that no 

 naturalists with whom I have conferred on this subject 

 have been able to determine with certainty the species of a 

 sponge by the description given by Lamarck in his ' Anim. 

 s. Vert./ or by those of any other author who has adopted 

 the same method of description, with the exception of, per- 

 haps, a few very striking species. 



The division of the Spongiadac by their chemical consti- 

 tuents may serve very well to separate them into primary 

 groups, but they are far too limited to be applied as generic 

 characters. I have therefore for this purpose rejected both 

 systems, and have retained the latter one for the purpose of 

 forming primary divisions only, and I purpose founding 

 the generic characters principally on the organic structure 

 and mode of arrangement of the skeleton. Spongilla differs 

 in no respect from Halichondria, as now accepted by natu- 

 ralists ; and the latter, even in the narrow circle of the list 

 of British species, contains at least ten distinct modes of 

 arrangement of the skeleton, each of which is constant and 

 well defined in its character. 



It is not my intention to propose the rejection of any of 

 the well-established genera of my predecessors, but to con- 

 fine each genus strictly within the bounds indicated by the 

 peculiar mode of the structure of the skeleton which exists 

 in that species of sponge which is the oldest-established 



