PORIFEKA. 179 



of doubt whether certain simple organised beings, such as the Sponges 

 and several so-called Alcyonetta, are animal or vegetable, the absence 

 of all voluntary motion in these bodies, whether of the whole or of in- 

 dividual parts of it, must determine the question ; and they must more 

 properly be numbered among the vegetable marine structures. It may 

 certainly be said, that the embryo of sponges (as Dr. Grant has shown), 

 like the embryo of polyps and corals, moves by means of cilia ; but 

 the distinctive marks between the embryo of sponges and marine In- 

 fusoria are by no means certain, and similar motions have been many 

 times observed in the embryo of true vegetables, of the alg89, for 

 example." 



Again, we read that " M. Dujardin, having repeated his observa- 

 tions on spongillae, or fresh-water sponges, as well as others on marine 

 sponges, thinks he has proved that these ambiguous beings are posi- 

 tively groups of animals, capable of contraction and extension. If a 

 piece be detached from a living sponge, and submitted to a microscope, 

 it will be seen to group itself into irregularly-rounded masses, and 

 change the form of its edges incessantly : isolated portions, detached 

 from the general mass, move slowly in the liquid, and creep along by 

 means of their alternate contraction and expansion." 



The description given of sponges by Dr. Johnston is, that they are 

 " organised bodies growing in a variety of forms, permanently rooted, 

 unmoving and irritable, fleshy, fibro-reticular, or irregularly cellular ; 

 elastic and bibulous, composed of a nbro-corneous axis or skeleton, 

 often interwoven with siliceous or calcareous spicula, and containing 

 an organic gelatine in the interstices and interior canals ; they are 

 reproduced by gelatinous granules called gemmules, which are generated 

 in the interior, but in no special organ. All are aquatic, and with few 

 exceptions marine." The same author says : " Mr. J. Hogg, in a letter 

 dated June 25, states that the green colour of the fresh- water sponge 

 (iSpongilla fluviatilis) depends upon the action of light, as he has 

 proved by experiments which showed that pale-coloured specimens 

 became green when they were exposed for a few days to the light and 

 full rays of the sun; while, on the contrary, green specimens were 

 blanched by being made to grow in darkness or shade. He therefore 

 infers the vegetable nature of this sponge ; but leans to the opinion 

 that the sea sponges are animals." 



Dr. Lankester has given a somewhat better definition for the deter- 

 mination of animal and vegetable life than other investigators. He 

 proposes that the general fact of vegetables giving off oxygen gas and 

 absorbing carbonic acid, whilst animals absorb oxygen and give off 



