42 ANATOMY AND PHYSIOLOGY 



scribed, and with which I am acquainted, where they per- 

 form the office of tension spicula only, they are destitute of 

 spines. In other cases the tension spicula not only fulfil 

 their own especial office, but they subserve that of defen- 

 sive spicula also. Thus in the dermal membrane of Spon- 

 yilla lacustris, Johnston, we find them dispersed rather 

 numerously, covered with short acutely conical spines, as 

 represented by Fig. 90, Plate IV. In Spoiigilla alba, Carter, 

 we find the tension spicula as abundantly spinous as those 

 of S. lacustris, but in this case the spines are truncated 

 (Fig. 91, same Plate). They have a similarly blunted im- 

 perfectly produced character in those of Pachymatisma 

 Johnsfonia, as represented by Fig. 93. 



The production of tension spicula in the membranes of 

 the Spongiadae is by no means a peculiarity of that class of 

 animals. We find them in numerous beautiful forms in the 

 skins of the Holothuriadas, varying in shape in the different 

 parts of the animal to adapt themselves to the necessities 

 of their situation ; but the closest approximation, both in 

 size and form, to those of the Spongiadas are the bihamate 

 ones that are found so abundantly dispersed on the mem- 

 branous tubular suckers of Echinus sphcera ; and I have 

 also seen another variety of these spicula in the tubular 

 tentacles of a large common species of Actinia ; and in 

 the latter case they were even more minute than those of 

 the Spongiadas. 



Retentive Spicula. 

 1st. Bihamate Spicula. 



In the interior of the sponge we find a series of retentive 

 organs in the various forms of bihamate and anchorate 

 spicula, which exist in large numbers attached to the 

 surfaces of the interstitial membranes. The simplest forms 

 of spicula of this kind are those of the bihamate, in which 

 we have an acerate form of spiculum, bent near each termi- 

 nation into the shape of a hook, the curves being either in 

 the same plane or at right angles to each other, and the 

 terminations being attenuated and acute. 



