RECENT ANB FOSSIL. 99 



secretes a homy case which it covers with grains of sand ; the Forami- 

 nifera, when it secretes a calcareous case ; and tlie Polycystina, when 

 it secretes siliceous cases or framework ; and this brings us back to 

 sponges, for, as we have already seen, we have the chalky sponge, 

 the flinty sponge, and the horny sponge, and all these varieties of 

 sponges are secreted or built up by a similar form of fleshy matter, 

 called sarcode.* 



Sponges obtain their food, and the horny, calcareous, or siliceous 

 matter necessary for the production of their frame-work, or skeleton, 

 by inhaling the surrounding water through pores in the dermal 

 membrane or skin ; and exhaling the same through large orifices 

 known as osculae. During this process of inhalation and exhalation 

 the sarcodous matter of the sponge is supposed to assimilate the 

 food necessary for its growth, in the same manner as do the more 

 simple amoeboid animalcules which we have been describing. 



It is very difficult to discover the mode by which this inhalation 

 and exhalation of water is effected; but, inasmuch as in a species 

 called Grantia compressa ciliary action has been satisfactorily made 

 out, it is inferred that ciliary action takes place in all cases, other- 

 wise it would be very difficult to comprehend how such very strong 

 currents of water could be forced through sponges as is known to 

 be the case. 



I have seen a freshly- gathered marine sponge, placed in a shallow 

 dish of water, produce a considerable elevation on the surface of 

 the water by the excurrent stream, the sponge being at a depth of 

 from half to three-quarters of an inch beneath the surface, and not- 

 withstanding the presence of this very powerful action, upon sub- 

 jecting the sponge to the microscope no cilia could be perceived. 



I find that in a paper in ' Science for All,'f Dr. J. Murie states 

 that the cilia in sponges " are not promiscuously dispersed, but are 

 confined to minute, deeply-situated chambers or dilatations of the 

 canals." These chambers are, he says, of very diminutive capacity, 

 and " are encircled with a closely-set series of flask-shaped cells or 

 bladders, sunk in the gelatinous, fleshy substance, a single lash-like 

 cilium protruding from each." I think it is very probable that 

 the reason so many microscopists have failed to observe the cilia is 

 the presumption that they lined the whole of the canals, and 

 thus these ciliary chambers were overlooked. Again, Mr. J. Fulla- 

 gar, writing in ' Science Gossip,';]: says that it is impossible to detect 

 cilia in living sponge, but when he examined a portion which was 

 torn off " cilia were plainly shown." He, however, says nothing 

 about ciliary chambers. 



There is, however, one stage in the life of sponges in which cilia 

 may be universally observed, viz. in those little buds or gemmules 

 by which the species is propagated. Here we find ciliary action 



* The sponge -structure is really built up by a number of very minute individuals 

 which have a close resemblance to some of the simpler flagellate Infusoria ; and 

 the Spongida may, perhaps, be most correctly considered to be a class of the 

 Protozoa nearly allied to the Infusoria, but lower, or simpler in organisation. — Ed. 



t Vol. i, p. 59. X Vol. xvi, p. 4. 



