40 THE VOYAGE OF H.M.S. CHALLENGER. 



aud incapable of practical applicatiou in many cases. While in the Lyssacina the 

 cementing of the spicules occurs at a relatively very late stage, and generally only 

 after the Sponge has attained its final form and size, the continuous framework of 

 the Dictyonina is formed at once in every newly developed portion, and constitutes from 

 the very beginning an important factor. 



The spicules of the Lyssacina are, during the entire period of growth, easily 

 separable from one another ; but when they become united by the cement into a 

 rigid framework, the growth of the sponge ceases. In the Dictyonina, on the other 

 hand, on account of the early development of the rigid framework, growth can only 

 continue through increase on the surface or at the ends, but there is as little definite 

 limit to this superficial growth as to the general growth of those Lyssacina in which the 

 spicules are never firmly united. 



It seems to me worthy of notice that, in certain divisions of the Lyssacina, as, e.g., 

 in the great family of the Hyalonematidse, and in the subfamily of the Holascinae, a 

 firm union of the spicules never occurs. It is certainly no accidental fact that it is in 

 these very divisions that the largest species occur, such as Poliopogon gigas, Poliopogon 

 amadou, Pheronema giganteum, Malacosaccus vastus. 



Position and Arrangement of the Spicules. 



The position and arrangement of the spicules now remain to be considered. A 

 definite grouping and naming of the spicules is of obvious advantage for description even 

 in those cases where no sharp distinctions exist between the various categories. I would, 

 therefore, endeavour to complete the classification and nomenclature already adopted by 

 Marshall. 



Prostalia. 



The more or less prominent spicules which occur over the outer surface of the Sponge 

 I call prostalia. They occur only in Lyssacina, and may, from their position, be more 

 intimately defined and differentiated as hasalia, pleuralia, and marginalia. 



Basalia. — The basalia are prominent spicules at the lower end of the Sponge, 

 grouped together like bunches of long hairs, and forming the root-tuft which serves for 

 anchoring the animal in the mud. This is one of the characteristic family peculiarities of 

 the Hyalonematidse, but it also occurs in many Euplectellidse and some Rossellidfe. 



When the knowledge of the Hexactinellida was still limited to a few forms, such as 

 Euplectella aspergiUum, Hyalonema sieholdii, and some Dictyonina, it was proposed to 

 utilise the presence or absence of a root-tuft as a leading principle of classification, and 

 to erect a special group of " LophospongiaB." In this, the importance of what is merely 

 an adaption to the nature of the ground, was over estimated. We now know, in fact, 



