and. Physiology of the Spongida. 17 



without foreign bodies in the core ; but when we find fibre 

 cored with foreign objects or " proper spicules," we have to 

 assume that the original germ, which for convenience we will 

 term " horn-cell " (in accordance with what has already been 

 stated of the horn-cell in the Hydractiniada?, whose functions 

 are analogous, 'Annals,' 1873, vol. xi. p. 6, pi. i. fig. 7), is at 

 first living and plastic, when, amoeba-like, it may take in foreign 

 bodies, and, having arranged them in a linear or branched 

 form, then proceed in the way mentioned to anastomose with 

 the branches of other similar horn-cells, and thus finally pro- 

 duce a reticulated structure with a core of foreign objects as 

 continuously throughout its whole course as the "fine granular 

 tube " in the fibre without foreign objects, subsequently in 

 like manner receiving, concentrically, additional layers from 

 the intercellular sarcode of the sponge. 



There is much more to commend this theory to our notice 

 in the microscopical examination of the fully developed fibre 

 itself, which need not be mentioned here ; at the same time, 

 it is impossible to conceive how foreign objects or " proper 

 spicules " can become the core of horny fibre unless by some 

 such hypothesis as that above stated. 



Having thus described the spicules and the fibre of the 

 Spongida, it is desirable to notice here that as there are sponges 

 which only possess siliceous spicules, these will be termed 

 " Siliceous Sponges," while others which only possess calca- 

 reous spicules will be termed "Calcareous Sponges." 



Again, as the fibre may be either glass-like with proper 

 spicules, or horny alone, or horny with foreign bodies, or horny 

 with proper spicules, or composed of spicules only, the terms 

 "vitreous," "horny," " arenaceo-horny," "spiculo-horny," 

 and " spiculo-fibre " will be used for these kinds respectively ; 

 while there is yet another modification, as before stated, in 

 which the core may consist of foreign bodies and proper 

 spicules mixed. 



Dissolution of Fibre and Spicules. 



I have already noticed the disappearance by wasting or 

 decay both in the siliceous and calcareous spicules, together 

 with that of the glassy fibre ('Annals,' 1873, vol. xii. p. 456 

 et seq.) ; but I omitted to notice that the " proper spicules " 

 of the spiculo-horny fibre also disappear after the same 

 manner, leaving nothing in many instances but their central 

 canals, with a fragment perhaps of the entire shaft in some 

 art of their course, frequently in the middle (thus looking 

 ike a cotton-reel upon long spindles), which at first appeared 



Ann. & Mag. N. Hist. Ser. 4. Vol. xvi. 2 



i; 



