20 



PKOTOZOA. 



own account just as well as when forming a part of the original animal- 

 cule. On cutting one of these creatures in two, or tearing it to pieces, 

 there is no escape of fluid perceptible ; but each portion contracts itself, 

 and commences a separate individuality. 



(43.) In one species of Amoeba (A. verrucosa, Ehr.) Mr. Carter* has 

 witnessed ovular development, the Amoeba perishing as the ovules are 

 perfected, and ending in becoming a mere ovisac. When first formed, 

 the ovules, which are spherical, consist of a hyaline capsule enclosing a 

 sphere of glairy, refractive fluid; but as they begin to increase, this 

 glairy matter becomes transformed into a granuliferous mucus which 

 is spread over the inner surface of the capsule ; and finally the granules 

 present motion whether of themselves or by the aid of the mucus in 

 which they are imbedded is uncertain. The history of their further 

 development has not yet been made out; but Mr. Carter thinks that the 

 next stage of their growth consists in the whole ovule becoming poly- 

 morphic. 



(44.) SPONGES. However dissimilar apparently, both in form and 

 structure, from the simple organisms described above, it is in their im- 

 mediate vicinity that we must place the extensive group of SPONGES, 

 which has until recently held a very dubious position upon the confines 

 of the animal and vegetable kingdoms. 



(45.) The common sponge of com- 

 merce is, as every one knows, made up 

 of horny, elastic fibres of great deli- 

 cacy, united with each other in every 

 possible direction, so as to form in- 

 numerable canals which traverse its 

 substance (fig. 9, c). To this struc- 

 ture the sponge owes its useful pro- 

 perties, the resiliency of the fibres 

 composing it making them, after 

 compression, return to their former 

 state, leaving the interstitial canals 

 open, to suck up surrounding fluids 

 by capillary attraction. 



(46.) The dried sponge is, how- 

 ever, only the skeleton of the fabric. 

 In its original state, before it was 

 withdrawn from its native element, 

 every filament of its substance was 

 coated over with a thin film of glairy, 

 semifluid matter that constitutes the 

 living part of the sponge, secreting, as it extends itself, the horny fibres 

 which are imbedded in it. 



Fig. 9. 



Spicula and horny skeleton of various 

 Sponges. 



* Ann. & Mag. Nat. Hist. 2nd ser. vol. xx. p. 37. 



