vSponges. 209 
branches. The oscula are small and scattered ; each is approached by a ramifying 
horizontal subdermal channel into the floor of which the main exhalent channels open. 
The colour is brownish or clay-coloured. 
The gemmules form a pavement-layer at the base of the sponge or are arranged 
in small groups which adhere tightly to the object to which the sponge is attached. 
Bach gemtnule is small (0 68 mm. in diameter) and subspherical and has a single 
foraminal tubercle, which is situated in the middle of the upper surface. The pneu- 
matic layer is fairly thick but rather irregular ; its cells are small but well-defined. 
The gemmules are of a dark brown colour. 
The skeleton is extremely compact and hard, resembling that of Potamolepis, 
Marshall : it consists of a close network of single spicules and bundles of spicules with 
interstices that are polygonal both in vertical and in transverse sections. There are 
Fig. 6. — Spicules of Sfongilla potamolepis, x 250. 
no well-defined spicule-fibres, but there seems to be a fairly, but diffuse secretion of 
horny matter at the nodes of the skeleton. There is very little if any inhalent sub- 
dermal cavity. 
The skeleton-spicules are all smooth and at least moderately stout but vary 
greatly in shape. In the older parts of the largest specimen I have examined the 
majority are amphistrongylous and often a little inflated at the extremities. In less 
well-developed sponges, though similar spicules can be discovered, the majority of the 
macroscleres are both longer and more slender ; they are still distinctly amphystrongy- 
lous but not inflated at the tips. Spicules of this type are gradually replaced towards 
the periphery of young sponges by araphioxi sometimes considerably longer than 
themselves. We may thus find a single sponge with spicules that are from 9 to 20 
times as long as thick. The longest amphioxi are about 0 32 mm. long and the shortest 
amphistrongyli 0 24 mm. long. There are no flesh-spicules. The gemmule-spicules, 
