AjN'D PHYSIOLOar of the sponoiad^. 
765 
which they form so important an element. When the areas of the reticulations are 
large, they are generally long and rather stout, and are usually shorter when the propor- 
tions of the network are small and close. When enclosed in keratose fibre, they are 
most frequently smaller and shorter in their proportions than those in the Halichondroid 
sponges. And in those species in which they are dispersed over the membranous tissues, 
as in Hymeniacidon, Boweebajvk, MS., they are generally long, slender, and frequently 
flexuous. In the sponges of this structure having siliceous spicula the triradiate form 
of spiculum occurs but rarely, while in the calcareous sponges, which consist of membranes 
and dispersed spicula, the triradiate forms of skeleton spicula are the normal ones. 
When the skeleton is constructed of large fasciculi of spicula, as in Tethea and Geodia, 
they attain their greatest dimensions as essential spicula of the skeleton, frequently 
exceeding the eighth of an inch in length. 
The greatest kno-wn length of spicula occurs in the prehensile ones of Euplectella 
aspergillum and cucumer, Owen, where they are found to exceed three inches in length; 
and in Hyalonema mirabilis, Geat, where, in the spiral column of the great cloacal 
appendage, they reach the extreme dimensions of six or seven inches in length ; but in 
both these cases the spicula must be considered as auxiliary and not essential forms. 
The larger number of forms of skeleton spicula are perfectly smooth, but in some 
species they are partially or entirely covered with spines. 
In every case they appear in the living state to have the capability of a change of 
position within the fibre to a considerable extent, in accordance with the natural altera- 
tions arising from the extensions or contractions of those tissues. 
The spicula are among the earliest-developed organs of the sponge. Dr. Geant, in 
his valuable “ Observations on the Structure and Functions of the Sponge,” published 
in the Edinburgh New Philosophical Journal, vol. i. p. 154, states that spicula are 
developed in the locomotive gemmules of Halichondria panicea [Hal. incrustans, 
Johnston) before they attach themselves for life and commence their development as 
hxed sponges. And in the gemmules of Tethea cranium they are abundantly developed 
even before the gemmules are detached from the parent, and some of them are of forms 
peculiar to the gemmule. 
The growth of the spicula and their mode of extension appear to vary according to 
circumstances. Thus an acerate spiculum is at first short and very slender ; as the deve- 
lopment proceeds it increases in diameter, and appears to lengthen equally from the 
middle towards both ends ; but in spinulate ones the increase in length does not appear 
to be effected in the same manner as in the acerate form, as we often find spinulate 
spicula fully developed at the base, while the shaft is exceedingly short and the apical 
termination hemispherical instead of acutely pointed, as in the adult state. As the shaft 
lengthens towards its full proportions, it attenuates ; but in all the intervening stages the 
apical termination is usually more or less hemispherical. The progressive development 
from the base to the apex of the spinulate form is beautifully illustrated in the skeleton 
spicula of a new and very singular British sponge from Shetland, Halicnemia patera^ 
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