AND PHTSIOLODY OF THE SPONG-IADiE. 
805 
recurved spines, it will be at once seen, are eminently fitted to retain annelids or other 
such prey, and to cause every motion of the struggling victim to contribute to its own 
laceration and destruction, while the structure and mode of attachment of the cruciform 
base is admirably calculated to resist the force and motions it has to sustain in such en- 
counters. But these spicula, although exceedingly numerous, are not the only organs 
capable of retaining intruders into the body of the sponge with which it is furnished : 
there are, in addition, numerous large multihamate birotulate spicula dispersed in various 
positions on the sides of the interstitial cavities of the sponge, each of the rotulee con- 
sisting of seven or eight stout recurved fiattened radii, which if immersed in any struggling 
animal would be capable of sustaining a vastly greater amount of force than many of 
the spiculated quadriradiate ones combined could endure without injury ; and that their 
especial ofiice is that of auxihary retentive organs is well demonstrated by the fact that 
the trenchant edges of the fiattened radii are all at right angles to the line of force 
requhed to tear away their hold of any body in which they may have been inserted. Thus 
they appear destined by nature to secure the prey, while its own struggles among the 
lacerating organs contribute to its destruction (Plate XXXI. figs. 3, 4, 5, 6 & 7). 
In the modification of the structure of the contort bihamate spicula, and their peculiar 
adaptation to the retention and destruction of intruders within the sponge, which I have 
described when treating on the internal defensive spicula, and which is represented in 
Plate XXXI. figs. I & 2, we have precisely the same physiological principle carried out, 
but by means widely different from those I have previously described. 
If we consider the whole of these extraordinary organs to which I have referred in 
relation to each other, we cannot fail to see that, however varied their forms may be, 
there is every appearance of perfect harmony of design in the purposes they are destined 
to efiect in the economy of the Spongiadae. 
The Cilia and Ciliary Action. 
Our knowledge of the cilia of the Spongiadae is, comparatively speaking, very small. 
Dr. Gkant is, I believe, the first author who has seen and described these organs in situ. 
This learned and accurate observer, in his paper “ Observations on the Structure and 
Functions of the Sponge,” has described the origin and gradual development of the ova 
or gemmules of Spongia jpanicea {Halichondria incrustans., Johnston). After the libera- 
tion of these bodies from the sponge, he writes, “ The most remarkable appearance exhi- 
bited by these ova is their continuing to swim about, by their own spontaneous motions, 
for two or three days after their detachment from the parent, when they are placed 
separately in vessels of sea-water, at perfect rest. During their progressive motions they 
always carry their rounded broad extremity forward, and when we examine them under 
a powerful microscope we perceive that these motions are produced by the rapid vibra- 
tion of cilia, which completely cover over the anterior two-thirds of their surface.” And 
he further states that they are “ longest and exhibit the most distinct motions on the 
anterior part, and that they “are very minute transparent filaments, broadest at their 
5 E 2 
