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



A thin ectosome, or dermal membrane, is usually associated with a reticulate dermal 

 skeleton, as in Pachychalina lobata {vide PI. XL VI. fig. 4); or it may simply be supported 

 on the ends of brushes of spicules, as already described when speaking of the skeleton. It 

 may perhaps be studied to the best advantage in the Homorrhaphidag, but is also weU 

 illustrated in the Axinellidse. Thus in Phakellia ventilabrum, var. connexiva (vide 

 PI. XLIX. fig. 3), it is everywhere a thin membrane, perforated on the one surface of the 

 sponge by the pores and on the other by the oscula, and in this case there is no dermal 

 skeleton at all. 



The mesodermal constituents of a thick ectosome are, as we have already indicated, 

 very various in nature. We have found it a very difficult matter to classify them, but 

 have finally decided to distinguish between stellate, amceboid, fibrous, vesicular and 

 glandular f These elements may be present in various combinations. 



It is probable that stellate and amceboid cells are present in greater or less quantities 

 in every ectosome, just as they are normally present in the ground tissue of the choanosome 

 between the flagellated chambers. When they are largely developed but unaccompanied 

 by any fibrous tissue, or with only a small proportion of it, they give rise to a thick, gela- 

 tinous ectosome, such as we have found in Spirastrella massa, nobis, Axinella\V}paradoxa, 

 nobis, and in the genus Esj^erella {Esperella gelatinosa, Eidley, Esperella murrayi, 

 nobis, and Esperella lapidiformis, nobis) ; in such cases there is, at any rate very often, 

 a thin outer layer separable from the remainder of the ectosome as a dermal membrane. 



As an example of a thick gelatinous ectosome we may take for more detailed descrip- 

 tion that of Esperella murrayi. In this sponge the ectosome varies much in thickness in 

 different places, interdigitating in an irregular manner with the choanosome (PI. XL VIII. 

 fig. 2), a condition which appears to be rather characteristic of the genus, and strongly 

 contrasting with the sharp distinction between • ectosome and choanosome in most of 

 those cases where the former forms a true cortex [Suberites caminatus, nobis, Stylocordyla 

 stipitata, var. globosa, &c.). Immediately beneath the epidermis, which consists of the 

 usual flattened epithelial cells, there is a thin, fibrous layer, usually about O'Ol mm. 

 thick, resting upon a strongly developed, reticulate dermal skeleton. Below the dermal 

 skeleton comes another thin, fibrous layer, resembling that just mentioned, and the soft 

 tissues between the two, in which the dermal skeleton is embedded, are also more or less 

 fibrous. Down to the bottom of the lower fibrous layer may be considered as " dermal 

 membrane," between the ends of the primary fibres it is about 0'14 mm. thick. 



The dermal membrane covering over the subdermal cavities {vide infra), is, however, 

 excessively thin, and contains no dermal skeleton; it is perforated by the pores (PL XL VIII. 

 fig. 2b), and in these areas the fibrous tissue is concentrated in special bands running 

 from side to side between the pores (PI. XIII. fig. 16, 6 ; PL XLVIIL fig. 2&,/). These 

 fibrous bands are composed of densely packed, very much elongated cells, with rather 

 faintly discernible nuclei scattered here and there. They do not run straight from side to 



