REVIEWS. 35^ 



Geodea, they attain their greatest dimensions as essential spicula of the skeleton, 

 frequently exceeding the eighth of an inch in length. 



" The greatest known length of spicula occurs in the prehensile ones of Euplec- 

 tella aspergillum siud cucumer, Owen, where they are found to exceed three inches 

 in length ; and in Uyalonema 7nirabilis, Gray, where in the spiral column of the 

 great cloacal appendage they reach the extreme dimensions of six ar 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 

 alterations arising from the extensions or contractions of those tissues. 



" The spicula are among the earliest developed organs of the sponge. Dr. Grant, 

 in his valuable ' Observations on the Structures 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 Ilalichondria panicea, {Hal. 

 incrustans, Johnston,) before they attach themselves for life and commence their 

 • development as fixed 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 forms peculiar to the gemmule. 



" The growth of the spicula and their mode of extension appears to vary accord- 

 ing to circumstances. Thus an acerate spiculum is at first short and very slender ; 

 as the development 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 propor- 

 tions, it attenuates; but in all the intervening stages the apical termination is 

 usually more or less hemispherical.'' 



His general account and subdivision of the auxiliary spicula is as 

 follows : — 



" Beside the spicula essential to the structure of the skeleton, there are several 

 other forms of these organs, many of which, although not absolutely necessary in 

 the structure of the skeleton, are of very frequent occurrence in subsidiary organs 

 found in peculiar species and in particular genera. They may be conveniently 

 classed under the following heads : 



Connecting spicula. 



Prehensile spicula. 



Defensive spicula. 



Tension spicula. 



Retentive spicula. 



Spicula of the sareode. 



Spicula of the ovaries and gemmules. 



