336 SECTIONAL TRANSACTIONS.— D. 



ecdyses recovered. Another reliable index is the number of pedunculate 

 bodies in Graber's organ, of which there are twice as many for any particular 

 instar as the number which designates that instar. 



Mr. C. F. A. Pantin. — The origin of body fluids. 



The evolution of a multicellular organism was necessarily accompanied 

 by the appearance of intercellular spaces. These spaces are cut off from 

 direct communication with the external medium, and the composition of 

 the fluid within them is therefore potentially under the control of the 

 organism. This has two important consequences : it permits chemical 

 intercommunication between different parts of the organism, and by 

 endowing the latter with a controlled internal medium it provides an ideal 

 environment for the tissues. Such an internal medium may be to a great 

 degree independent of the external environment. 



It is from this internal medium that the blood of the higher organisms 

 is developed. The organism can only maintain the composition of its 

 internal medium by continuous physiological activity. Substances pass 

 into the blood from the outside world by way of the body surfaces, the 

 respiratory organs and the gut, and to these substances are added the 

 products of metabolism. On the other hand, loss of substances takes place 

 continuously by the same channels and by the excretory organs. It is 

 evident that the blood cannot be in passive equilibrium with the external 

 medium. 



In studying the origin of the conditions which maintain the composition 

 of the body fluids, investigation may be turned towards the Turbellarian 

 worms. In these there is no blood system, and the internal medium is 

 restricted to the intercellular spaces. The Triclad Procerodes ulvce is able 

 to withstand vast changes in the composition of the external medium. 

 By studying the conditions which enable it to do this, light is thrown on the 

 nature of the mechanisms controlling the internal medium. 



Saturday, September 3. 



Excursion to Bylands and Rievaulx. 



Monday, September 5. 



Dr. C. Tate Regan, F.R.S. — Some results of the Dana expeditions. (At 

 the Rialto, Fishergate, by the kind permission of the proprietor.) 



Dr. Stanley Kemp, F.R.S. — Oceanography in the Antarctic. (At the 

 Rialto, Fishergate, by the kind permission of the proprietor.) 



A film illustrating work on the Discovery II was shown. 



Prof. A. C. Hardy. — Plankton research in the service of the fishing industry. 

 (At the Rialto, Fishergate, by the kind permission of the proprietor.) 



Earlier work was reviewed. Experiments with an instrument, the 

 Plankton Indicator, carried out on a number of different fishing grounds by 

 herring drifters and patrol ships, were described. During i93oand 1931 over 

 five hundred records of the quantity of fish caught were obtained, together 

 with samples of plankton taken at the same time. A positive correlation 



