SPONGIAE1A 



73 



together by spongin or connective tissue fibres or articulate together to 

 form a network, and constitute the supporting framework of the body. 

 The latter are minute flesh spicules of a variety of forms which are scat- 

 tered throughout the tissues. 



The cellular elements of the mesoglea fall into two distinct groups: 

 (1) those which are derived from the ectoderm, and (2) the archeocytes. 

 The former migrate into it and are either scleroblasts which secrete the 

 spicules, the spongioblasts which secrete the spongin fibres, or the col- 

 lencytes or connective-tissue cells which are distinguished by their stel- 

 late form and thread-like pseudopodial processes. The archeocytes are 

 primitive cells derived from the blastomeres during development, which 

 perform a variety of important functions. They are amoeboid cells 

 which are nutritive in function, ingesting and digesting food, and also 

 supply a circulatory element in that they aid in distributing nutriment. 

 They also give rise to the reproductive elements the spermatozoa, the 

 ova, and the gemmules. 



The entoderm con- 

 sists uniformly of col- 

 lar cells in all sponges. 

 The flagella of these 

 cells do not act in 

 unison, but each for 

 itself, and have for 

 their main function the 

 creation of currents in 

 the water which bring 

 the animal food and 



oxygen and carry away the wastes. The current thus produced enters 

 the pores, traverses the radial canals and flagellate chambers into 

 the cloaeal cavity and passes out again through the osculum. The 

 food consists of organic particles and minute animals and plants; 

 these are ingested and digested by the collar cells in the lower, 

 calcareous sponges, but in the greater majority of them principally by 

 the amoeboid archeocytes. No special excretory or respiratory organs 

 and no muscles, nerves or sense organs are present in sponges, although, 

 as we have seen, contractility is present in the ectodermal cells, and sen- 

 sitiveness to external stimuli is often noticeable. 



Sponges have three methods of reproduction: (1) by budding, (2) by 

 the formation of gemmules, and (3) by sexual methods. Budding is 

 simply growth which results in the formation of new oscula, each oscu- 

 lum representing a new individual ; it is of very general occurrence. In 

 a few sponges the bud becomes separated from the parent sooner or 



Fig. 134 'Spicules of sponges (Minchin). 

 scleres j B, microscleres. 



A, mega- 



