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



(b) Pores arranged in more or less definite Pore-areas. 



We may consider this part of our subject under tliree heads, according to whether the 

 pores arc (1) localised in areas owing to the arrangement of the dermal skeleton, 

 (2) localised in areas owing to the arrangement of the subdermal cavities, the dermal 

 skeleton being either absent or so feebly developed as not to interfere with their 

 arrangement ; or (3) localised in areas owing directly to the habit of the sponge. 



(l) Pores localised in areas oiving to the arrangement of the dermal skeleton. — The 

 simplest cases of this kind (if such they can be called) are those in which the dermal 

 skeleton is well developed and regularly reticulate, and the surface is broken up hj the 

 fibres of the reticulation into a number of polygonal areas, in which the pores are 

 scattered. Such a condition we find in Pachychalina lobata (PL XLVI. fig. 4), l)ut, as a 

 glance at the figure will show, these cases might be classed almost as well under the 

 heading of " Pores scattered." 



We have, however, to consider certain very remarkable cases in which, omng to the 

 arrangement of the dermal skeleton, the pores are collected into j^erfectly definite, 

 confined areas, strongly contrasted with the remainder of the surface of the sponge. We 

 shall describe this condition as it occurs in four distinct s^^ecies belonging to four very 

 difi"erent families, viz., Halichondria latruncidioides, Esperella murrayi, Latruncxdia 

 ajncalis, and Tentorium semisuberites. 



In Halichondria latruncidioides (PL I. fig. 5) the dermal skeleton is composed of a 

 dense, continuous layer of large oxeote spicules, placed in such a manner (PL XLVI. 

 fig. 5) as to leave no interstices, or very few, through which it would be possible for 

 water to obtain entrance to the sponge. The consequence is that the pores are collected 

 together into perfectly definite, round or oval, raised pore-areas (PL I. fig. 5, p.a.), from 

 which the dermal skeleton is absent and in which they are so abundant as to reduce the 

 dermal membrane to a mere sieve (PL I. fig. 5a). In the genus Halichondria the dermal 

 skeleton is usually reticulate and the pores irregularly scattered or placed in the meshes 

 of the reticulation. Our sponge agrees with the other species of the genus in all 

 essentials, and we are obliged to consider the peculiar arrangement of the pores as a case 

 of special adaptation, consequent upon the unusual density of the dermal skeleton over 

 the general surface. This view of the case is supported by the fact that, where there do 

 happen to be odd gaps in the almost continuous skeleton, we sometimes find j^ores in 

 them, although they are nearly all confined to the special raised areas. This case of 

 adaptation has an unusual interest owing to the curious resemblance which the species in 

 question bears to another sponge coming from the same locality (Station 320), viz., 

 Latrunculia apicalis, and careful investigation has shown us that the similar external 

 form of these two species is due to the same cause in both, as will be seen later on. 



