PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 



93 





Elephant's ear sponge 



Venus's flower-basket 



Glass rope 

 sponge 



Finger sponge 



Fresh-water sponge 



Figure 45. Some types of fresh-water and marine sponges showing various shapes. The figures 

 are not drawn to scale. 



LEUCOSOLENIA-A 

 SIMPLE SPONGE 



Leucosolenia is a simple sponge (Fig. 46), 

 whitish or yellowish in color. It is attached to 

 rocks at the seashore, just below low-tide 

 mark, and consists of a number of horizon- 

 tal tubes from which branches extend up 

 into the water. These branches have an 

 opening, the osculum, at the distal end, 

 and buds and branches project from their 

 sides. The cavity within each branch is 

 known as the central cavity (spongocoel). 

 A large number of three-pronged (triradi- 

 ate) spicules are embedded in the soft tis- 

 sues of the body wall; these serve to 

 strengthen the body and hold it upright. 



The body wall (Fig. 47) is usually said 

 to consist of two layers of cells: an outer 



layer composed of dermal amoebocytes,* 

 and an inner layer consisting of flagellated 

 collar cells (choanocytes), with mesoglea 

 (often jelly-like material) between, in which 

 are many amoeboid wandering cells. Ac- 

 cording to de Laubenfels, the term ''layer 

 of cells" must be used with reservations, for 

 no true epithelial layers of cells exist in the 

 sponge, such as are present in other meta- 

 zoan animals. The outer and inner lavers 

 appear to be firm in sponges that are hard- 

 ened in fixing solutions, but in life there is 

 a mesoglea, sometimes nearly as liquid as 

 water, and again almost cartilaginous in 

 density, with cells crawling through it or 

 clustered on its external and extensive inter- 



* These cells are so termed by de Laubenfels be- 

 cause he reports that the cells on the external sur- 

 face of the sponge are actually amoebocytes which 

 do not occupy any one permanent position. 



