312 THE VOYAGE OF H.M.S. CHALLENGEK. 



Thomson as characteristic of the species AphrocaUistes heatrix, Gray. This, which 

 has been figured by me in PL LXXXIV. figs. 9, 10, occurs irreguhirly scattered in 

 great numbers throughout the whole parenchyma. While the one axis of the spicule 

 has two rays greatly prolonged, and while, each of these two long rays divides into 

 four diverging, pointed terminals, the rays of the two other axes, which cross the first in 

 the middle, remain simple short principals, which end in sharp points. The entire form 

 may be therefore described as a longitudinally extended oxyhexaster, in which the four 

 short princij)al rays remain undivided, while each of the principals of the long axis, which 

 are sometimes provided with lateral prickles, divides into four diverging, pointed terminals. 



Since parenchymalia of this kind are not found in any other form of AphrocaUistes, 

 it becomes possible to determine the separation of this form as a distinct species ; I must, 

 however, draw attention to the fact that in AphrocaUistes hocagei, which is also very 

 similar in external appearance, I found widely scattered parenchymal oxyhexasters, and 

 similar forms were also figured by Oscar Schmidt in the Spongien des Meerbusens von 

 Mexico (pi. vi. fig. 3). They do not indeed completely agree with the above peculiar 

 spicules, but they approach them, and evidence at least a close affinity between the two 

 forms. The view which Oscar Schmidt has expressed,^ to the effect that the spicule 

 w^hich is characteristic of AphrocaUistes heatrix is an accidentally introduced foreign 

 element I cannot accept. 



For purposes of comparison with the other species of the genus AphrocaUistes more 

 carefully described below, I will here give a short summary of the most important 

 microscopic skeletal characters, based upon my examination of the original specimen of 

 AphrocaUistes heatrix, Gray. 



The dictyonal framework is formed, as represented by Wy\dlle Thomson ^ and by 

 Bowerbank,' of a tolerably irregular, narrow-meshed network with strongly thickened 

 crossing knots. The beams are almost entirely and more or less thickly beset with small 

 tubercles, but these tubercles are stronger and more abundant on the spherical nodes of 

 intersection, and on each of the strong conical pegs which project freely both on the 

 dermal and gastral surfaces, and also in the interior of the radial prismatic mesh-spaces. 

 The pegs, projecting into the lumen of the mesh-spaces, seldom stand exactly at right 

 angles to the surface of the reticulate partition from w^hich they spring, but are directed 

 obliquely inwards towards the gastral cavity of the entire sponge. The middle portion 

 of all the septa between adjoining prismatic radial canals consists only of a single layer 

 of irregularly fused hexacts, and an irregularly triangular prismatic interspace is formed 

 where three such septa meet. 



The dermal skeleton consists of delicate hexacts in which the distal ray bears 

 numerous narrow, curved, fir-tree-like, lateral prickles, while the five other straight 



' Spongien des Meerbusens von Mexico, p. 50. ^ ^^yi_ g^^yi Mmj. Nat. Hist., ser. 4, vol. i. p. 123. 



^ Proc. Zool. Soc. Land., 1869, pi. xxi. figs. 2-4. 



