16 THE BRITISH SPONGES. 



disposed to assent. It is, indeed, evident that, from the 

 unequal densities of the mucilaginous secretion of the 

 sponge and the circumfluent water, there must be unceas- 

 ingly going on an oozing out of the one, and an entrance 

 inwards of the other, in obedience to the law in ques- 

 tion, which Dutrochet discovered to regulate the trans- 

 mission of fluids through all organic membranes. By this 

 singular process of reciprocal exchange we account proba- 

 bly enough for the exhalation of the excretions of the 

 sponges, and for the admittance of the circumfluent water ; 

 nor is more necessary for their sustenance and growth, 

 since, like plants, they appear to live solely on water and 

 its mineral ingredients. This liquid food is not received 

 into any cavity, but permeates to all points, and is equally 

 elaborated in every part of the system, which, in one sense, 

 is an unconfined digestive cavity, where the various ingre- 

 dients are separated, selected, and fitted for appropriation 

 by each species agreeably to its nature. For example, it 



as albumen or solution of gum, the weight and turgescence were still 

 more increased, and their increase was more rapidly completed ; in eight 

 hours and a half, a caecum partially filled, and weighing 58 grains, be- 

 came extremely turgid, and weighed 130 grains. This transmission of 

 the water by inward impulse or Endosmose, exists to a certain, but much 

 inferior degree, when the caecum is empty. It occurs always when the 

 internalfiuid is more dense than the external." " When, on the contrary, 

 the caecum was filled with rain-water, and immersed in any of the above- 

 mentioned active fluids, such as milk or albumen, the water passed out- 

 wards through the membrane. In like manner, a weak solution of gum- 

 arabic passed outwardly towards a stronger solution. The last-mention- 

 ed facts are examples of Exosmose or impulse outwards But the phe- 

 nomena of Endosmose and Exosmose are always concurrent or recipro- 

 cal ; that is, in each of the examples hitherto given, the external fluid 

 passes inwards, while the internal fluid passes outwards." See an excel- 

 lent review of Dutrochet's Discoveries in Physiology, in Edin. Med. and 

 Surg. Journal, xxxi. p. .38.3. 



