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



Suberite which we have described under the name Cliona dissimilis (p. 227; PL XXV. 

 fig. 5, &c.). Here the sponge has bored its way into a flattened coral which it completely 

 surrounds; hence it has itself acquired a flattened, lamellar form, and we find the pores 

 collected in areas (woodcut. Fig. XL, pa), on one side of the sponge and the oscula 

 (woodcut. Fig. XL, o) on the other. 



There is no other known example, so far as we are aware, of a lamellar Suberitid 

 sponge, and even the species in question is lamellar only because it has bored into a 

 lamellar coral, and yet the pores and oscula are arranged just as they would be in a free 

 living, frondose sponge such as Phakellia. There must be some strong reason why, as 

 soon as a sponge, for any cause, acquires a lamellar form, the oscula become confined 

 to one surface and the pores to the other, and to account for the occurrence of this 

 condition in genera so widely separated as Gellius, Myxilla, Phakellia and Cliona. 

 What this reason may be we cannot at present say. 



(2) The Subdermal Cavities. 



The subdermal cavities are the spaces into which the pores directly lead, and from 

 which the inhalent canals proper take their origin. They might be regarded merely as 

 the proximal portions of the inhalent canals, but inasmuch as they are sometimes very 

 distinctly marked ofi" from the remainder of the inhalent canal system, both by position 

 and structure, it is convenient to treat of them separately and to retain for them a special 

 name. Although the term subdermal is hardly a suitable one to apply to these structures in 

 corticate sponges, as we shall see better later on, yet it is advisible to have the same term 

 for the same structures in the two groups Halichondrina and Clavulina, and as the term 

 subdermal cavity is already in use we shaU retain it in preference to inventing a new one. 



In the Halichondrina the subdermal cavities are usually expanded horizontally and 

 more or less lacunar in form, being roofed in only by the thin, pore-bearing dermal 

 membrane, and many pores usually lead into one and the same subdermal cavity. The 

 extent to which they are developed varies much in difi"erent sponges, attaining its 

 maximum in the genus Ciocalypta amongst the Axinellidse. Here, it will be remem- 

 bered, the thin dermal membrane is supported at some distance from the choanosome on 

 slender pillars of spiculo-fibre, and the numerous pores lead into one large, continuous 

 subdermal space, interrupted only by these supporting pillars.^ In certain other 

 Axinellidee, e.g., Phakellia, the subdermal cavities are also largely, but not nearly so 

 largely developed, but this not does appear to be a constant family character. 



In most of the Halichondi-ina, however, the subdermal cavities are rather vague and 

 ill-defined, and distinguishable from the remainder of the lacunar canal system more by 

 their position than by any peculiarities in structure ; they show no great regTilarity either 

 ^ BowerLank gives a good figure of this arrangement in Mon. Brit. Spong., vol. i., pi. xxx. fig. 360. 



