486 



CHARACTERS OF INVERTEBRATE ANIMALS 



in the case described, and the collar-cells are restricted to small 

 rounded chambers situated in the thickness of the wall and 

 communicating by narrow canals with the exterior on the one 



hand and the central 

 cavity on the other. 

 The canal system 

 may be exceedingly 

 complex, as may be 

 realized by looking 

 at the labyrinth of 

 spaces present in 

 an ordinary bath- 

 sponge. 



It will be suffi- 



Fig. 299. -Bath-Sponge (Em&mfia), reduced cient for the prCSCttt 



purpose to consider 



the Sponges as divided into two groups according to the nature of 

 the skeleton; i.e.: i. Calcareous Sponges, and 2. Siliceous Sponges. 



1. Calcareous Sponges, when simple, correspond to the type 

 described. A more complex example is afforded by the common 

 British species Grantia compressa, a small white flattened sponge 

 about an inch long, frequently found attached to stones and other 

 objects between tide-marks. Other forms are represented in fig. 

 300. 



2. Siliceous Sponges include a great variety of forms in which 

 the supporting spicules, when present, are of siliceous or flinty 

 nature. One of the commonest British species is the Bread- 

 crumb Sponge (Halichondrid), which, in the form of yellowish- 

 brown or orange-coloured masses, may be found encrusting rocks 

 near low-tide mark. On the surface of the sponge a number 

 of little conical elevations may be seen, and on the end of each 

 an osculum is situated. The pores by which currents enter are 

 placed in the part of the sponge between these projections. For 

 beauty of form nothing can surpass Venus s Flower - Basket 

 (Euplectella), a deep-water form in which the skeleton may be 

 compared to a cornucopia with a wall of lace -like appearance 

 and a perforated lid. Another very interesting kind is the 

 Glass-rope Sponge (Hyalonema) dredged in deep water off Japan. 

 Here the ovoid body is anchored in the mud by bundles of long 

 twisted spicules which have suggested the name. 



