Mr. H. J. Carter on Specimens 



Lithistina. 



In describing the Lithistids it is absolutely necessary to 

 have specimens which possess the last-formed dermal layers 

 in addition to a portion of the fully formed internal structure, 

 because these are the parts which are most characteristic of 

 the species ; hence, although we may not possess the fully 

 developed entire form, the thinnest layer, provided it contains 

 the parts mentioned, will be sufficient to determine the species ; 

 for these will ever be the same, although the adult form of 

 the sponge itself may be different. So that, while the speci- 

 mens on the Melobesian nodules of the Gulf of Manaar are so 

 small that, comparatively, they hardly amount to much more 

 than traces of structure which may attain a large and definite 

 form in the deeper sea, still, so far as they go, they will enable 

 us to predict what they may attain in that situation. 



For the most part, they have grown over the layers of 

 Melobesia from which the nodules have been chiefly formed, 

 while in many instances they themselves have been over- 

 grown by one of the Microcionina that have been described ; 

 but, whether overgrown or not, as the spiculation of a Lithis- 

 tid, for the most part, is so locked together that even boiling 

 in pure nitric acid does not separate its parts, so in this way 

 it has been easy to free the Lithistid not only from the calca- 

 reous Melobesia on which it rests, but from the Microciona 

 covering it, to such an extent as to cause it to come out under 

 this treatment in a clean, perfect, and beautiful form. I 

 have stated " for the most part," because it may be easily 

 conceived that the elements of which a Lithistid is composed 

 are not inextricably locked together until they have under- 

 gone a certain amount of development, and therefore, being 

 more or less united by sarcode until this occurs, they are, up 

 to this time, separable by boiling in nitric acid. Such is 

 more particularly the case with the Lithistids whose structure 

 commences in disks (viz. the Discodermioz), which disks we 

 shall hereafter find to be gradually transformed into their 

 branched and complicated spiculation. But even here, under 

 the boiling in nitric acid, all the separated parts can be easily 

 retained, and, when mounted in Canada balsam, present, 

 when thus separate, a much better view of their gradationary 

 development than when in situ, where they lie more or less 

 obscured in layers one over another as they are formed. 

 The term " interlock " is here used advisedly ; for whereas in 

 the vitreous Hexactinellida the spicules are cemented together 

 by the addition of glassy fibre, no such thing occurs in the 

 Lithistida, whose spicules are united by mere apposition of 

 the expanded ends of their filigreed branches (PI. VIII. 



