AOT) PHYSIOLOGY OF THE SPONGIADiE. 
803 
cavities, and may thus form subsidiary defences to those organs. Although emanating 
from the fasciculi of the shafts of the true connecting spicula, their form, slender proportion 
and position evidently indicate a different office from the spicula with which they are 
associated, and no other purpose for them occurs to me so probable as the one I have 
suggested above. Or we may carry the supposition further, and believe them to be not 
only defensive but aggressive organs ; also, like the recurvo-quaternate spicula, their 
office may be to retain soft annelids that have intruded themselves through the oscula 
into the digestive organs, to aid in the nutrimentation of the sponge ; and this idea 
appears the more admissible as these spicula are never observed in the intermarginal 
cavities, where the decomposition of animal matters would be offensive to their especial 
function, but always in the spaces beneath them, which are the commencements of the 
digestive system. 
The same course of reasoning will apply to their occurrence in such considerable 
quantities amidst the defensive fasciculi of spicula projected from the surface of Tetliea 
simillimus, and also of T. crania, the latter being represented in Plate XXIX. hg. 12 c, 
in which it mil be seen that the recurvo-ternate heads of the spicula are always situated 
beneath the level of the true defensive spicula. Thus situated they would form an 
admirable trap for the entanglement of soft annelids that might attempt to crawl over 
the surface of the sponge, and thus they would be destroyed and retained for the imbi- 
bition of their particles liberated by their gradual decomposition. If this be not their 
especial purpose in this situation, I must confess myself at a loss to imagine their proper 
function, as the surface of the sponge is effectually protected by the porrecto-ternate and 
large acute spicula that compose the defensive fasciculi projecting in such abundance 
from all parts of the sponge. If we also consider the structure and positions of the 
ordinary forms of internal defensive spicula, the entirely spined attenuato-acuate ones, in 
reference to the idea of their being offensive as well as defensive organs, we shall not 
fail to see that, although less striking in their forms and modes of disposition than the 
spicula already described, they are calculated to subserve the office of retaining prey 
quite as effectually as the more singular ones. The abundance in which they occur, the 
vast number of spines with which they are covered, the apices of which are frequently 
long and recurved, combined with the mode in which their bases are attached to the 
fibres of the skeleton, exhibiting a beautiful combination of strength and flexibility, are 
strongly indicative of a purpose beyond that of mere repulsion. 
In the two species of sponges in which are found the acuate entirely and verticillately 
spined defensive spicula in situ, represented in Plate XXX. figs. 1 & 8, one of them 
has the spicula collected in groups in a manner very similar to those of the spinulo- 
recurvo-quaternate form, and if the latter be considered as organs for the retention of 
prey, the physiological purpose of the grouping together of the former can scarcely 
be considered in any other light. 
In the isolated positions of these forms of spicula, viewed in reference to some ideas 
regarding their physiological purposes, there are some circumstances of a very remark- 
MDCCCLXII. 5 E 
