No. 3-] DEVELOPMEXT OF MARINE SPONGES. 283 



scarcely stains at all and consists of a mere coating for the 

 nucleus, prolonged at opposite ends into slender protoplasmic 

 processes. Such cells are especially abundant in the mesoderm 

 of the dermal membrane, Fig. 4. The edges of pores and 

 oscula are always provided with such cells, which here are 

 especially long and fibre-like, and serve as a support for the 

 free edge. The mesodermic bands which support the dermal 

 membrane (Fig. 4), and the partition walls found in branching . 

 canals (/. w. Fig. 7) contain great numbers of these fibre-like 

 cells. The ectoderm and the epithelioid lining of the canals 

 are formed of flat cells. 



The disposition of the peripheral skeletal bundles is shown 

 in Fig. 2. The bundles are composed of spicules, such as that 

 shown in Fig. 3 c? (oxytylotes, Sollas), which project slightly 

 from the surface of the dermal membrane. Fig. 2. Besides 

 forming bundles, the oxytylotes are scattered in abundance 

 throush the mesoderm, but are not united into a meshwork. 

 A variation from the ordinary type of spicule is occasionally 

 found, with a head like that shown in Fig. 3 b. After boiling 

 in caustic potash, some spicules are always found with the 

 pointed end split as in Fig. 3 <r, doubtless an effect due to the 

 action of caustic potash. The bow-shaped spicules. Fig. 3 d 

 (toxaspires, Sollas), the s-shaped spicules, Fig. 3/(sigma-spires), 

 and the sigmas. Fig. 3 c, are all of about the same size, the 

 first form being less abundant than the other two. They are 

 found scattered about in the mesoderm in all parts of the 

 sponge. 



The large shovel-shaped spicules, of which a face view is 

 given in Fig. 3"i7, and a side view in Fig. 3" ^, are rare. 

 Shovels of about half the size, of which Fig. 3'/; gives a face 

 view and Fig. 3' c a side view, are comparatively abundant. 

 When the spicule is viewed more or less from the end, Fig. i' a, 

 it is seen that the shovel shape is an illusion, that the blade of 

 the shovel is not flat, but is a figure of three dimensions. If 

 an oval body should be divided by a transverse plane, passing 

 through a point on the equator and a point on the opposite 

 surface somewhat nearer one of the poles, two parts would be 

 obtained, of which the smaller would roughly correspond in 



