40 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



oscula are plainly visible ; situated in Halichondria at the apex 

 of tall crater-like prominences ; less marked in Clathria, where they 

 crown low rounded swellings, and in Pachymatisma, peculiar as 

 being collected into special elevated areas, and there arranged in 

 serried, closely set rows. Details of histology must, for want of space, 

 remain over to a future article, and with this reservation, I will 

 now proceed to deal with the four species in question. 



Halichondria panicea (Johnston), the Crumb of Bread, and also 

 the Coxcomb Sponge of Ellis, varies much in colour, ranging from 

 a greyish yellow to various shades of green. It is noteworthy that 

 the higher up the littoral and the more exposed to light, the greener 

 becomes this sponge. The yellow and ash coloured varieties are, 

 conversely, found in the lower zones of the littoral or where matted 

 curtains of fucus cause obscure shadowy light. In this sponge, 

 the entire duty of keeping open the canals devolves upon the 

 spicules, no fibrous tissue being present. The spicules (Fig. 1 a) are 

 all of one form, curved pointed rods, arranged in a well ordered 

 meshwork. Around the larger passages, the rows become much 

 strengthened. Sub-dermal cavities are greatly developed. 



Clathria seriata (Johnston), the Ophlitaspongia seriata of 

 Bowerbank, shows more complexity in its skeleton. A well developed 

 tough, square-meshed, horny network supports the sponge, beset, 

 urchin-fashion, with numerous stout smooth rodshaped spicules 

 pointed at one end only (smooth styli). The meshwork arrangement 

 of spicules seen in Halichondria is absent, and apparently the 

 horny skeleton is here used to keep open the canals — the spicules 

 ceasing such function and limiting their benefit to defence against 

 would-be enemies. A second and slender form of spicule, bow-shaped 

 (toxon) can also be made out. 



Pachymatisma johnstonia (Bowerbank) grows to immense 

 proportions, and probably is the most massive of British sponges. 

 The one photographed was 6 inches long, weighing just half-a-pound. 

 Others we have had, have much exceeded this, e.g. one which is now 

 deposited in the Guernsey Museum, weighed when alive fully 6-lbs., 

 and measured 12-ins. x 7 — 9-ins., and 4 inches high. In section, 

 this sponge shows many interesting points. An outer thick crust, 

 cortex or ectosome is greatly developed by the massing of immense 

 numbers of sterrasters — rounded and oval spicules thick set with 

 spines radiating in all directions (4 d). Similar spicules are spread 

 throughout the inner tissues of the sponge, the subcortical or 

 choanosome region, but if examined carefully, it will be seen that 

 these are much more sharply spinous than those of the cortex, which 

 appear worn and battered. Probably these spicules are formed in the 

 choanosome, and then pass outwards to accumulate in the cortex as a 



