PHYSIOLOGICAL SERIES. 



mental tissue, becoming greatly elongated and often 

 branched. Their walls are thickened by a secondary 

 deposit of lignone (Ci 2 H 18 9 ) on the inner side of the 

 primitive cell-wall formed of cellulose (C 12 H 20 0] ). Nume- 

 rous crystals of calcium oxalate are imbedded in the outer 

 surface of these cells. The spicular cells appear to confer 

 some extra strength and stiffness to the parts in which they 

 are present. They may also render the plant distasteful to 

 animals ; and perhaps serve as a mode of disposal of 

 redundant carbohydrates. (Fig. 2.) 



Royal Gardens, Kew. 

 Hooker, Trans. Linn. Soc v vol. xxiv. 1864, p. 1. 



ANIMALS. 



PoKIFERA. 



A. 6. A Calcareous Sponge (Grantia compressd). Its soft 

 parts are supported by definitely arranged calcareous 

 spicules whose forms are shown in the drawing. Each 

 spicule is formed in one of the cells of the mesoderm. 

 In this sponge they consist of calcium carbonate, having 

 the crystalline structure and other properties of calcite; 

 the form, however, is organic. 



Presented by Prof. C. Steivart. 



A. 7. A Siliceous Sponge (Craniella \_Tetliya\ cranium), belong- 

 ing to the Sub- Class Tetractinellida. The spicules do not 

 fuse ; they are composed of colloid silica in layers around 

 a delicate central cavity occupied by organic matter. The 

 large spicules (megasoleres) are mostly arranged in bundles 

 radiating from the centre of the sponge and projecting 

 slightly on its surface. Minute flesh-spicules (microscleres) 

 are also present. Presented by Prof. C. Stewart. 



A. 8. Half of a Siliceous Sponge (Corallistesnoli-tangere. Sub- 

 Class Tetractinellida). The soft tissues and loose spicules 

 have been removed, leaving only the main skeleton. It 

 consists of branched solid spicules (desma) that are 



