78 RICHARD EVANS. 



small. The membranes which line the canals are not pro- 

 vided with special spicules, but are supported by the spicule 

 fibres, which are situated close to the lining membranes. 



4. The Structure of the Gemmule. — The gemmules 

 are scattered about singly throughout the whole tissue of the 

 sponge. They are found, on the one hand, near the surface, 

 and on the other hand, quite close to the vegetable supports 

 of the sponge. They are never found in groups. Each gem- 

 mule occupies its own cavity (PI. 1, fig. 2, gem.). 



I shall here describe only the structure of the mature gem- 

 mule, the development of which will be described in Part II. 

 Nevertheless, it must be remembered that the sponge con- 

 tained gemmules in all stages of development at the time it 

 was collected. 



The gemmule is oval in shape, being, as a rule, slightly 

 flattened on the side on which the opening is situated. The 

 external opening or pore is placed at the bottom of a small 

 depression surrounded by a rosette-like structure, which is 

 raised up, and into the composition of which all the layers of 

 the gemmule coat enter (PI. 1, fig. 7). 



The contents of the gemmule consist of a number of glo- 

 bular cells which are full of oval-shaped food granules. The 

 cells are all alike, and the whole mass possesses no membrane 

 of any kind save the gemmule coat, which I shall now proceed 

 to describe. 



The gemmule coat consists of three layers which differ 

 from one another, to a considerable degree, both in structure 

 and extent of development. 



The inner layer of the gemmule coat completely surrounds 

 the cells which are situated in the interior. It presents the 

 general shape of the gemmule and is prolonged round the 

 aperture to form a kind of a tube, the passage through which 

 is interrupted by a chitinous membrane situated about the 

 middle. The cellular contents of the gemmule extend into 

 the inner half, and the second layer of the gemmule coat to 

 the outer half of this tube. In structure this layer is chitinous, 

 and resists the action of all ordinary reagents, save the mineral 



