Invertebrate Gallery of the Indian Museum. 13 



the other. The tube-wall is built up of three layers of 

 cells (1) a very thin outer layer of delicate cells known as 

 the ectodermal epithelium, {2) an inner layer of large cells 

 known as the endodermal epithelium, and (3) a middle 

 layer of cells and intercellular substance secreted by the 

 cells, known as the mesodermal connective tissue, or 

 sponge parenchyma. In this mesodermal tissue a sup- 

 porting skeleton of horn, or lime, or flint is usually de- 

 veloped, and it is this skeleton, washed and dried, that 

 constitutes most of the Museum specimens of sponges. 



The hollow of the tube is known as the " gastral cavity," 

 and its open end as the "osculum". 



The osculum does not correspond to the mouth of other 

 Ccelenterates. The sponge in fact does not possess a 

 mouth, but the body-wall is perforated by large pores 

 which can be closed and opened again. Through these 

 pores currents of water stream into the gastral cavity^ 

 carrying with them the microscopic organisms which form 

 the food of the sponge, and passing away out through the 

 osculum. 



This simple type of sponge is seen in Ascetta, which, 

 however, is not represented in the collection. Such simple 

 forms are uncommon. In the Syconidae, of which several 

 specimens {Sycandra^ Grantessa, Leucandra) are exhibited 

 in the collection (Case 1), the mesodermal parenchyma 

 gets much thicker, and becomes tunnelled by pouch-like 

 offshoots of the gastral cavity, the ends of the pouches 

 opening to the exterior by pores, and causing the conical 

 bulgings to be seen on the outside wall of the sponge 

 where they open. This not very complicated type is fairly 

 well seen in the exhibited specimens of Sycandra and 

 Sycon, both of which are shown in longitudinal section, in 

 Case 1. 



In most other cases the structure of the sponge is still 

 more complicated by the further thickening of the meso- 

 derma parenchyma and the repeated branching of the 



