130 PHYSIOLOGY. 



The product of tliis process, consisting of the dissolved and disintegrated nutritious portion 

 of the food, mingled with certain materials which it has received from the walls of the digestive 

 cavity, and diluted with water which has been introduced from without, is propelled from the 

 cavity of the hydranth into that of the coenosarc, or from the manubrium into the radiating canals 

 of the medusa, in order to be distributed to the several tissues. It thus becomes the somatic fluid, 

 to be presently considered in its relations to the circidatory functions. 



Examined under the microscope, this fluid is seen to be of a very heterogeneous nature. Its 

 basis is a transparent colourless liquid, and in this solid bodies of various kinds are suspended. 

 These consist partly of disintegrated elements of the food, partly of solid coloured matter which 

 has been secreted by the walls of the somatic cavity, partly of cells, some of which have undoubt- 

 edly been detached from these walls, though it is possible that others may have been primarily 

 developed in the fluid, and partly of minute irregular corpuscles, which are possibly some of the 

 effete elements of the tissues. 



Beyond this point it is plain that digestion cannot be traced as a specialised function, and its 

 phenomena here become coincident with those of circulation. 



2. Circulation, Nutrition, and Respiration. 



Between the phenomena associated above under the genei'al head of digestion, and those 

 which might with equal justice be claimed by circulation, it is impossible to draw any well- 

 marked line of demarcation. The somatic fluid, whose relations to digestion we have been just 

 considering, has relations quite as intimate with circulation ; for though in one aspect we may 

 compare it to the chyme or chyle of the more highly specialised animals, in another it admits of 

 just as close a comparison with the blood. 



No trace of a differentiated blood vascular system has been detected in any hydi'oid. The 

 nearest approach to it is that which is presented by the radiating and circular canals of the 

 medusa, and yet these are simple oS'sets from the digestive cavity. 



The place of the blood is taken by the fluid which pervades the somatic cavity, and which 

 consists of the digested food largely mingled with water which has gained admission by 

 the mouth, as well as with certain materials which have been secreted by the walls of the 

 somatic cavity. 



The fluid which thus permeates the somatic cavity and extends through all its ramifications 

 must not be supposed to be in a state of rest. On the contrary, it is subjected to constant 

 motion, which manifests itself in currents more or less regular in their velocity and definite in 

 their direction.' In some cases the currents present a remarkable definiteness and regularity. 



mouth. 1 have not been able to confirm this observation ; and though Leydig (" Ueber den Bau der 

 Hydren," in ' Miiller's Archiv,' 1854) and Hancock ("Notes on a Species of Hydra," in 'Ann. Nat. 

 Hist.,' 1850) support Corda's statement, I cannot avoid believing that the orifice in question is 

 accidental. 



' Cavolini was the first who distinctly noticed the currents of the somatic fluid in the Hydroida. 

 He has described them in a eampanulariau liydroid (Sprengel's 'Cavolini,' p. 56). They were subse- 



