OF THE SPONGIAD^. 15 



In every case they appear in the living state to have the 

 capability of a change of position within the fibre to a con- 

 siderable 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 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 gem- 

 mules of Halicondria 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 according to circumstances. Thus an ace- 

 rate spiculum is at first short and very slender ; as the de- 

 velopment 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 termina- 

 tion 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. 

 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, Bowerbank, represented by 

 Pigs. 228, 229, 230, 231, 232, and 233, Plate X. Pig. 

 230 represents a short variety of the normal spinulate forin. 

 In Pig. 328 we have a bi-spinulate, and in Pig. 229 a tri- 

 spinulate, form. The latter two are not mere malformations, 

 but they prevail to a great extent in the structures of the 

 sponge, subject to variations in the distances in the deve- 

 lopment of the second and third inflations from the basal 



