16 ANATOMY AND PHYSIOLOGY 



one. Figs. 231, 232, and 233, represent immature spicula 

 in progressive stages of development, the apices having 

 hemispherical terminations. 



Auxiliary Spicula. 



Beside the spicula essential to the structure of the skele- 

 ton, there are several other forms of these organs, many of 

 which, although not absolutely necessary in the structure of 

 the skeleton, are of very frequent occurrence in subsidiary 

 organs found in particular species and in peculiar genera. 

 They may be conveniently classed under the following 

 heads : 



Connecting spicula. 



Prehensile spicula. 



Defensive spicula. 



Tension spicula. 



Retentive spicula. 



Spicula of the sarcode. 



Spicula of the ovaries and gemmules. 



In the above designations of the auxiliary spicula, it 

 must not be understood that their respective titles strictly 

 define their offices, as it frequently occurs that under pecu- 

 liar circumstances the same form of spiculum is destined to 

 serve two, or even three, distinct purposes. Thus, an ex- 

 ternal defensive spiculum will occasionally perform reten- 

 tive offices for the purpose of securing prey; or internal 

 defensive spicula will combine the offices of defensive 

 spicula against the larger and more powerful of their ene- 

 mies with that of wounding and securing their smaller ones. 



The Connecting Spicula. 



These spicula are not necessarily a part of the skeleton ; 

 they are a subsidiary portion of it, occurrring under special 

 circumstances in a few genera only, such as Geodia, Pachy- 

 matisma, and other sponges which have a thick crustated 

 surface, which they serve to support and retain in due 



