336 SECTIONAL TRANSACTIONS.— D. 



than 1 c.c. per litre of sea water at night to over 9 c.c. per litre during the day. The 

 temperature changes are also very considerable. It is probable, however, that large 

 as they are, these changes have no harmful efiect on corals. 



One condition which has been supposed to have a considerable influence on coral 

 growth, is the degree of turbidity. Contrary to what is generally found in coral seas, 

 the sea between the Barrier Reef and the mainland is not clear. On an average it 

 is not more clear than the English Channel. As a result of this and also because of 

 the movement of sediment by wave action, corals are subjected to a continuous rain 

 of sediment. It has been found, however, that even quantities of sediment greatly 

 in excess of that moved by the waves can easily be removed by the common species 

 of corals. 



Mr. F. S. Russell and Miss S. M. Marshall. — The Plankton of the Seas 

 round the Great Barrier Reef. 



In view of the importance of plankton organisms as the main link between the 

 potential food reserve of dissolved nutrient salts in the sea and the reef building 

 corals themselves, it was necessary that a very complete survey of the plankton 

 should be part of the programme of the Great Barrier Reef expedition. Collections 

 were made weekly to obtain information on the changes in abundance and com- 

 position of fhe plankton throughout the year. The study of the centrifuge plankton- 

 was undertaken by Miss S. M. Marshall, and that of the zooplankton by Mr. J. S. 

 Colman and Mr. F. S. Russell. The results indicated that although poor compared 

 with the great abundance to be found in our northern seas, the plankton of the 

 Barrier Reef lagoon channel was a true coastal plankton and considerably richer 

 than that of the open ocean. The quantities of diatoms present showed no seasonal 

 changes comparable with the great outburst in our own seas known as the spring 

 and autumn increases. There was some slight indication that the richest period of 

 the year coincided with the periods of strongest winds, possibly caused by the 

 stirring up of the nutrient salts from the sea bottom. The animal plankton showed 

 no really great changes in total bulk, but its composition changed markedly at times. 

 There were, for instance, sudden outbiu-sts of Salps at certain times of the year, 

 whose appearance coincided with certain phases of the moon. Most of the species 

 of Siphonophores also disappeared during the rainy season when there was a definite 

 decrease in the salinity of the sea water. 



The most numerous group in the animal plankton was the copepods which 

 appeared in great numbers but were mostly very small. The difficulties of drawing 

 a true comparison between the abundance of plankton life in the barrier reef waters 

 and that of northern seas is increased when the difference in rates of development 

 between cold and warm water living animals is taken into consideration. Owing to 

 high rate of development in tropical waters periodic collections probably give too low 

 a figure. 



Dr. C. M. YoNGE. — The Food, Feeding and Digestive Processes of Corals. 



Dk. S. M. Manton and Dr. T. A. Stephenson. — The Growth, Breeding 

 and Life Conditions of Corals. 



Friday, September 5. 



Dr. D. de Lange. — Some Remarks on the Phylogeny of the Placenta. 



All criteria used for classifying the different forms of the placenta have a 

 descriptive and, strictly speaking, not a morphological significance. The same end 

 stages can be reached by different ways of development and need not be homologous. 

 Probably primary oviviviparous conditions have given rise to the formation of the 

 egg-membranes (chorion and amnion). In young stages the exocoelom forms the 

 protective water-cushion ; in later stages this function is performed by the amnion 

 cavity. The chorion (diplotrophoblast) originally possessed respiratory, and after- 

 wards acquired digestive significance. The trophoblast corrodes the maternal wall 

 and an ectoplacenta is formed which shows a histiotrophic {=lytic) character. This 



