PHYLUM PORIFERA 



93 



low-tide mark, and consists of a number of horizontal tubes from 

 which branches extend up into the water. These branches have 

 an opening, the osculum (osc), 

 at the distal end, and buds 

 and branches (div) projecting 

 from their sides. The buds 

 and branches are hollow, pos- 

 sessing a single gastral cavity 

 (Fig. 59, A, GC) which com- 

 municates with the horizontal 

 tubes. The entire mass is a 

 colony of animals, and the 



tissues connected with a 



, , , FIG. 52. A small colony of Leucoso- 



smgle osculum may be con- knia> a simple sp onge. osc, osculum ; 



sidered an individual Sponge. div > s . ide branches. (From Lankester's 

 T( . , , Treatise on Zoology.) 



If a branch is examined 



under a microscope, it will be found to contain a large number 

 of three-pronged (triradiate) spicules, which are embedded in 



the soft tissue of 

 the body- wall (Fig. 

 53) ; these serve to 

 strengthen the body 

 and hold it upright. 

 The application of 

 acid results in the 

 dissolution of these 

 spicules and the 

 production of an 

 effervescence, thus 

 proving them to be 

 composed of cal- 



FIG. 53- Leucosolenia, a simple sponge. View of cium Carbonate, 



a branch showing the sieve-like membrane (i) which The body-wall is 

 stretches across the osculum. The lower part shows n- f i , - f 



spicules only. (From Shipley and MacBride, after a 



Minchin.) difficult to Study 



