ETJSPONGIA. 133 



yellow, but varies according to the size or age of the sponge. The largest 

 specimens are very dark in colour, while smaller ones are of a light 

 melange. In spirit, the colour is fairly well preserved, but becomes duller 

 and lighter. Beach-worn skeletons diiJer very much in colour. Some varieties 

 are dark dirty greyish brown ; some have a remarkable orange-brown colour, 

 similar to burut sienua, and others are light yellow. 



The surface of all the varieties has the same structure. It is covered 

 with small, distally rounded conuli rather irregularly disposed, on an average 

 1 millim. high aud 2 millim. apart. Often two adjacent conuli coalesce, 

 whereby the irregularity in the configuration of the sui'face is much increased. 

 Sometimes there are zones 3 millim. in breadth without conuli, I'uuning up one 

 side of the digitate processes in a longitudinal direction. The oscula are scat- 

 tered over the outer surface of the sponge in an irregular manner. In those 

 specimens which possess aconulous zones, we always find the oscula in these 

 zones. The oscula are circular, and measure from 1-4 millim. in width. Slightly 

 magnified, the surface presents the appearance of a very regular network. 

 The meshes of this network measure from 0*l-0-4 millim. in width. In E. i. 

 silicata they are the largest (0'4 millim.), aud in E. i. jacksoniana the smallest, 

 measuring only 0-1 millim. This network is composed of a system of projecting 

 bands on the surface, which are about as broad as the concave fields between 

 them. Generally an abundance of sand-grains and other foreign material is 

 found in them, so that the whole structure attains the appearance of a very 

 regular sand-net. Long siliceous spicules are found in it in great numbers ; 

 these are of course foreign, collected, and not produced, by the sponge. 

 The depressed portions of the surface between this network are the pore- 

 sieves — thin unprotected membranes perforated by the inhalant pores. The 

 reticulate structure of the surface is developed in a similar manner in the fields 

 between the conuli and in the aconulous portions, as also in the surface of the 

 vestibular spaces. It is absent only in the surface of the ti'ue canals of the 

 sponge, and consequently a structure very useful in determining which canals 

 are true canals and which are vestibular. 



The canal-system of all the varieties is the same. In each pore-sieve 

 there are one or two large inhalant circular or oval pores, measuring on an 

 average 0-03 millim. in diameter. Their size is variable, but I have never 

 observed them entirely closed, although the strong muscular cells disposed in 

 rings around them, which form true sphincters, can contract them to a quarter 

 of their ordinary width. The largest pores observed by me measured 

 0-05 millim., the smallest 0*012 millim. I'rom these pores narrow canals 

 lead in an oblique direction down through the thick skin ; these canals have 

 an average width of 0*04 millim., which measurement corresponds to the 

 measurement of the dilated pores as seen from the surface. The skin which 



