110 ANATOMY AND PHYSIOLOGY 



while on spots perhaps immediately adjoining where the 

 dermal membrane is occupied by a thickly interwoven mass, 

 a felting of spicula, the probability is that not a single pore 

 can be detected. 



In some of the West India fistulose sponges we find the 

 large or primary area of the dermal surface composed of 

 keratose fibre, and within these large areas the dermal 

 membrane is strengthened and supported by a secondary 

 reticulation of spicula, in the areas of which the pores are 

 opened. In these secondary reticulations the spicula are 

 abundant, while in other parts of the sponge the tension 

 spicula are rather of rare occurrence. In Grantia, a sponge 

 of a widely different construction to those of the Hali- 

 chondroid type, they occupy the distal extremities of the 

 large intermarginal cavities of the sponge, and they appear 

 to open over the whole of those portions of the cavities not 

 in contact with the adjoining ones. 



In Pachymatisma Johnstonia, Bowerbank, a British 

 sponge closely allied to the genus Geodia, we find the 

 dermal membrane perforated by innumerable pores, some 

 as minute as -^^a inch in diameter, while others attained the 

 size of 5^ inch. They are nearly equidistant from each 

 other, but without any order in their arrangement. Imme- 

 diately beneath the dermal membrane there is a stratum of 

 membranous structure and sarcode destitute of ovaries, and 

 about equal in thickness to one third of that of the whole of 

 the dermal crust, the remaining two thirds of which consists 

 of a stratum of ovaries closely packed together, but per- 

 forated at intervals by the intermarginal cavities. Through 

 the upper stratum, destitute of ovaries, a small canal 

 passes from each pore to the nearest adjacent intermarginal 

 cavity, so that there are a series of them at various angles, 

 all concentrating their streams of inhaled fluid at the distal 

 end of the cavity, which is gradually expanded in diameter 

 to receive them. In these sponges, therefore, each mouth 

 appears to be furnished with a separate oesophagus, if I 

 may be allowed the term, connecting it with a stomach- 

 like cavity, common to a group of mouths above it; a 

 system of organization strikingly in unison with that of the 



