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



of the cortex which we have already described (p. xxxi). The pores (PI. L. fig. 3a, ])) 

 lead by narrow canals through the outer layer of the cortex into expanded, elongated 

 cavities in the middle layer of the cortex, and these on reaching the lower layer of the 

 cortex again contract into narrow canals (PI. L. fig. 3a, i/), which at the commence- 

 ment of the choanosome open into the much wider inhalent canals (PI. L. fig. 3a, i.e.). 

 This is the most complex type of subdermal cavity with which we are acquainted.^ 



These are the principal modifications of the subdermal cavities which are met with in 

 the Monaxonida. It will be seen from what has been said that the term is incapable of 

 a sharp definition, and that the sul:)dermal cavities of one sponge do not necessarily 

 correspond strictly to those of another. In the Halichondrina all we can say of them by 

 way of definition is that they are the spaces beneath the dermal membrane into which 

 the pores directly lead, while in the Suberitidse they are usually represented by so much 

 of the inhalent canal system as is enclosed within the ectosome ; and in both cases they 

 may or may not be sharply marked off from the remainder of the inhalent canal system. 



(3) The Inhalent Canal System below the Subdermal Cavities. 



In this section of the canal system we meet with but little variation amongst the 

 Monaxonida. As a very general rule the inhalent canal system, upon entering the 

 choanosome, becomes converted into a system of lacunar spaces, rather than definite 

 canals. Such we have found to be the case in the genera Halichondria, Reniera, 

 Esperella, Myxilla, Axinella, Latrunculia. If, however, as in the genus Esperella, 

 there is a thick, gelatinous ectosome, then the inhalent canals may, after leaving the 

 subdermal cavities, remain perfectly definite while in the ectosome and run straight down- 

 wards to the choanosome, where they at once break up into a system of lacunse. This 

 condition is very well illustrated in Esperella murrayi; each inhalent canal (PI. XLVIII. 

 fig. 2, i.e.) leaves the horizontally elongated subdermal cavity by a narrow exit 

 (PI. XLVIII. fig. 2a, i.e.) and dips vertically downwards towards the choanosome, upon 

 arriving at which it is at once lost in a system of irregular inhalent lacunge lined by 

 flagellated chambers. Another interesting feature of the inhalent canals in this sponge 

 is the manner in which their walls are guarded by projecting spicules (PI. XLVIII. 

 fig. 2c), as we have already described. 



In Latruneulia apicalis the lacunar arrangement is also well illustrated ; here we have 

 no long, straight canal interpolated between the subdermal cavity and the choanosome, as 

 in Esperella murrayi, but the subdermal cavity into which the pores lead passes directly 

 into a very wide, more or less lacunar channel (PL LI. fig. 1), from which numerous 



' Dr. Vosmaer has also described and figured the subdermal cavities in Tentoriuvi semisuberites (Sponges of the 

 " Willem Barents" Expedition, 1880-81, p. 19, pi. iii. fig. 22, &c.). His description agrees in the main with ours, but he 

 appears to have overlooked the important division of the cortex into three distinct layers. 



