282 THE VOYAGE OF H.M.S. CHALLENGER, 



The dermal membrane of Farrea occa is supported by the four rectangularly 

 intersecting tangential rays of simple pentacts, whose fifth rays penetrate into the 

 parench}Tna at right angles to the surface of the tube, while here and there a tubercle or 

 knob-like distal elevation remains as the persisting trace of a sixth (distal) ray. While 

 the proximal ray is always perfectly straight and slightly narrowed into a conical form 

 towards the pointed end, the four tangential rays are usually bent gently inwards, and end 

 in a slight usually blunted thickening, or in a somewhat knob-shaped swelling (PI. LXXI. 

 fig. 6). The proximal ray usually exceeds the tangential in length, is always cylindrical, 

 and generally smooth with the exception of a slight roughness at tlie point. The tangential 

 rays, on the other hand, are somewhat flattened on their outer side, and beset with 

 smaller or larger tubercles which are sometimes pointed, sometimes rounded and boss-like. 

 These tubercles or spines are usually strongly developed only on the gently convex and 

 somewhat flattened side ; on the lateral edges they are more weakly developed, and are 

 altogether absent on the inner side of the tangentials. In some cases they have an 

 isolated occurrence on the internal side. Towards the end of the rays the spines 

 increase in height, and occur more densely and all round. 



The disposition of these pentacts in the dermal membrane is frequently after this 

 fashion, that the quadratic framework, which is formed from their apposed tangential 

 rays, corresponds exactly to the subjacent dictyonal framework, each proximal ray being 

 apposed to a distal prominence of the latter, and the tangential rays lying parallel to the 

 beams of the dictyonal lattice-work. The meshes of the dermal lattice-work thus of 

 course correspond in size to those of the dictyonal framework. 



In other cases, as for instance in that on which the figures on PI. LXXII. fig. 5 and PI. 

 LXXIII. fig. 1 were based, each distal prominence of the dictyonal framework corresponds 

 indeed to the proximal ray of a dermal pentact, but besides this there is above the middle 

 of each mesh of the dictyonal lattice-work a dermal pentact; and the tangential rays of 

 all the dermal pentacts do not lie parallel to the beams of the dictyonal lattice, but 

 diagonally to the quadratic or rectangular meshes. Often enough, however, no definite 

 relation between the dermal pentacts and the dictyonal lattice can be determined except 

 this, that above most of the distal rays of the latter there occurs a pentact of the 

 dermal skeleton. The extent to which the corresponding opposite tangential rays of two 

 adjacent dermal pentacts may overlap one another, varies in the difi"erent regions of the 

 sponge. Each tangential ray frequently reaches almost to the nodes of intersection of 

 the neighbouring pentact (PI. LXXII. fig. 5). 



Besides the pentacts above described, the dermal skeleton of Farrea occa contains 

 clavulee, which are so far in connection with the former since they always lie close beside 

 the proximal ray. While they form an externally divergent tuft, the inferior pointed ends 

 of the long stalk are usually in contact with the proximal ray of the related pentact, and 

 the heads with their convex terminal discs lie about the level of the dermal membrane or 



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