EUSPONGIA. 127 



tons) a diameter of 10 millim. In the other varieties the oscula rarely 

 exceed 2 millim. in diameter. The oscula are confined to the upper surface or 

 the inner side of cup-shaped forms in the three varieties above mentioned, in 

 which the vents are large and conspicuous. In the other forms, E. o. nitens, 

 &e., they are scattered more or less irregularly over the whole of the surface. 

 In E. 0. lamella concavities 4-6 millim. wide, into which oscular tubes of small 

 size open, are met with on the surface. These depressions, which are sur- 

 rounded by frills, are apparently homologous to the large oscular tubes of the 

 first three varieties. Lipostomous forms without apparent oscula are rare : 

 I have observed a few such in the variety E. o. rotunda. The number and dis- 

 tribution of the large oscula in E. o. adriatica, E. o. molUssima, and E. o. rotunda 

 is subject to great variations. Sometimes a single large osculum, the oscular 

 tube leading up to which is divided by septa, is observed in the centre of the 

 upper surface. Often a few large and irregularly scattered oscula are found in 

 the upper surface, scattered or in groups. In E. o. rotunda the oscula are 

 very often arranged more or less regularly in longitudinal rows, which in 

 some individuals become exceedingly regular, prominent, and conspicuous. 

 In these the oscula in the rows are found not only on the upper surface, but 

 also on the sides, of the sponge. These lateral oscula often appear (in the 

 skeleton) very elongate, extended in the direction of the ridge on which they 

 lie, and slit-shaped, 4-8 millim. broad and 10-25 millim. long. The oscula 

 are surrounded by sphincter membranes, which are very muscular and assist 

 in regulating the water-current. 



The colour of the living sponge is dull. No bath-sponge has a bright 

 colour. E. 0. adriatica and E. o. mollissima are lighter coloured than E. o. exigua 

 and E. o. rotunda, whilst the colour of E. o. nitens appears particularly variable. 

 It varies from light yellowish brown in young individuals of E. o. adriatica 

 to dark brown in the same variety when adult and in E. o. molUssima ; and to 

 black in E. o. rotunda. The upper surface is generally darker coloured than 

 the lower, which difference is similar to that observed in the Chondrosida), 

 and attributable to a photographic chemical effect of the daylight, which, 

 illuminating the upper side more than the lower, turns this a darker hue than 

 the other, which is always in the shade. The colour in the interior is lighter 

 than on the surface, often yellowish, sometimes rust-red. 



The dry skeletons are yellowish to dark brown ; their colour depends to a 

 great extent on the method of maceration. The bath-sponges in use are 

 generally bleached artificially, and much lighter in colour than beach-worn or 

 simply macerated skeletons. E. o. irregularis is conspicuous by the brio-ht 

 straw-colour of its dry skeletons. E. o. rotunda has also a light-coloured 

 yellowish-brown skeleton. The other varieties have brown skeletons ; that of 

 E. 0. exigua is particularly dark — dark brown or dirty grey. 



