TYPE CCELEXTEKA. 73 



Nerve-cells Lave also been described, though a definite 

 nervous system cannot be said to exist. Elongated retractile 

 processes have been observed projecting from the surface of 

 certain sponges, and at the base of each is a group of stellate 

 cells each of which sends a long slender prolongation into 

 the process. To these cells a nervous function has been at- 

 tributed and the processes have been considered sensory ; 

 with the exception of these structures, however, no sense- 

 organs or nerve-elements have been yet observed. 



The Sponges may be arranged according to the nature of 

 their skeleton, in four orders. 



1. Order Calcarea. 



In the Calcarea the skeleton is always present and is 

 formed of spicules consisting of carbonate of lime. The 

 group contains forms of various complexity of structure, from 

 the simple cylindrical Leucosolenia constructed upon the 

 Ascon type through Sycon forms such as Grantia to repre- 

 sentatives of the third and fourth types. Indeed it is only in 

 this group that the Ascon and Sycou types of structure are 

 found. All the known species are marine and live at only 

 slight depths. 



2. Order Cornacuspongise. 



The skeleton of the Cornacuspougise consists either of 

 siliceous, needle-like spicules, freqiiently more or less united 

 by spongiolin, or else entirely of a network of fibres com- 

 posed of the latter substance. Like the Calcarea they are 

 inhabitants of shallow water and are for the most part marine, 

 though some forms (Spongilla, Fig. 37, Ephydatia) live in fresh 

 water. These fresh-water forms are of a green color due to 

 chlorophyll, the presence of this pigment being supposed by 

 some observers to depend on numerous unicellular algse liv- 

 ing in the substance of the Sponges. To this group belongs 

 also the Sponge of commerce (Euspongia), whose value de- 

 pends upon the entire absence of siliceous spicules, and 

 which is found in the shallow waters of the eastern portion 

 of the Mediterranean, in the Bed Sea, and in the Western 

 Hemisphere in the waters surrounding the Bahama Islands. 



