xxxiu 



THE PERIOASTRIC CAVITY. 



The space intervening between the outer wall of the 

 alimentary canal and the inner wall (or endocyst) of the 

 cell is known as the perigastric cavity. It is filled with a 

 colourless fluid, in which a large number of floating cor- 

 puscles of various size and shape are always present, and 

 which at certain times swarms with active spermatozoa. 

 Within this cavity the reproductive organs, both male 

 and female, are located ; and here the ova pass through 

 certain stages of their course, or, in many cases, complete 

 their development into the larva. 



Great interest attaches to the various questions relating 

 to the precise nature and physiological import of the peri- 

 gastric fluid, which bathes continually the body of the 

 polypide and the tissues which line the cell. In the ab- 

 sence of any direct analysis, it may be presumed that it 

 consists of water more or less charged with the products 

 of digestion*, and that it is subservient to the functions 

 of respiration and nutrition. But it is equally difficult 

 to determine precisely how the water finds access to the 

 closed sac, and how the chyle escapes into it from the 

 stomach. As to the first point, no orifices have been de- 

 tected through which the water might pass into the in- 

 terior of the cell, if we except the one for the escape of 

 the ova in Farrella, described by Van Beneden, and the 

 " intertentacular organ," investigated by Dr. Farre and 

 myself, which occurs only in a limited number of species, 

 and not constantly in these. 



* Beichert has made a rough analysis of the fluid contained in the inter- 

 nodes of the stem of Zoobotryon, and finds it to be a somewhat concentrated 

 solution of common salt (like the water of the Adriatic, where the species 

 was obtained), with traces of an albuminous substance (Abhandl. d. konigl. 

 Akad. d. VVissensch. zu Berlin, 159. phvsikalisohe Klaase, p. 268). 



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