780 
DE. J. S. BOWEEBAl^ ON THE ANATOMY 
Metentive Sjpicula. 
In the first part of this paper (Phil. Trans. 1858, p. 300) I have described the varieties 
in form and modes of development of these spicula. However varied they may be in 
form, when they are in their normal positions their ofiice appears “to be purely retentive. 
They are generally produced singly, and are dispersed without any approach to regularity 
over all parts of the sarcodous membranes of the sponge, abounding in some situations 
to a very much greater^ extent than in others. Their positions on, and mode of attach- 
ment to, the membrane are exceedingly varied, but in almost every instance it is such as 
to render the spiculum obviously subservient to the retention of the sarcode on the mem- 
branes which it covers. In one instance only I have found the simple bihamate spicula 
congregated in loose fasciculi. In this sponge, a new and very interesting species, 
Hymedesmia Zetlandica, Bowerbank, MS., they occur in great profusion. Very few of 
them occur singly ; nearly the whole of them are found in rather loose fasciculi, and the 
number is generally so great in each as to render it very difficult or impossible to count 
them. The mode of their disposition in the bundles is symmetrical, all the hami being 
in the same plane and coincident in dhection, as represented in Plate XXXI. fig. 8. A 
few bundles of reversed bihamate spicula were observed, and these in like manner were 
coincident in every respect like the simple bihamate ones. 
'When these forms of spicula are equal in the amount of the development of their 
terminations, and when their hami or palms are coincident in plane and direction, their 
normal mode of attachment is at the middle of the bow of the shaft, and the direction of 
their projection is at right angles to the plane of the membrane on which they are 
situated, so that both terminations are rendered effective as retentive organs, as repre- 
sented in fig. 8, Plate XXXI., dispersed on the membrane. But when their terminations 
are in different planes, or unequal in amount of development, then the normal mode of 
attachment to the membrane is by one end of the spiculum, while the other end is pro- 
jected into the sarcode above at various angles. This mode of disposition of the inequi- 
anchorate form of spiculum is beautifully illustrated in Halichondria lingua^ Boweebank, 
MS., a new species of British sponge from the Hebrides. 
In this case, as in Hymedesmia Zetlandica, we find these organs congregated, but in a 
very much more symmetrical and beautiful mode. They occur in rosette-shaped groups ; 
the smaller palms being adherent to the membrane in a circular form, and disposed as 
close to each other as possible, while the larger palms radiate from the centre at angles of 
about 20 or 30 degrees from the plane of the membrane beneath, as represented by 
fig. 9, Plate XXXI. 
I have selected this group for representation in consequence of its containing but a 
small number of spicula, and thus displaying the mode of arrangement more distinctly 
than a greater number would have done. In many cases these groups contain so large 
a number of spicula as to render any attempt to count them ineffectual, and in some 
instances so many are developed that the group assumes the form of a ball rather than 
