SECTIONAL TRANSACTIONS.— D. 335 



collected by the ' Discovery ' Whaling Expedition was found to be so well fixed that 

 it has been possible to work out in detail the anatomy of the chief systems of organs. 



The chief interest in the blood system lies in the presence of a muscular pericardial 

 floor and a system of blood vessels provided with muscles by which they can be 

 distended or collapsed. In addition, there is a pair of muscular valves of peculiar 

 structure at the point where the main vessels open into the body cavity. It is sug- 

 gested that this muscular system is an accessory circulatory mechanism, and from 

 a study of the arrangement of the muscles it has been possible to suggest the actual 

 mechanism by which the blood is forced round the body. The heart does not appear to 

 be essential and this is significant from the fact that in many Ostracods the heart 

 is absent. 



The compact nervous system is peculiar in that a basal ganglion occurs in each 

 limb. In addition, a well-defined system of giant nerve fibres occurs, most of which 

 are, as usual, associative elements, but some of which are apparently motor neurones 

 running to the powerful muscles of the swimming antennae. The structure of the 

 protocerebrum and deuterocerebrum has been worked out in detail. The trito- 

 cerebrum is peculiar in that a ' central body ' occurs in the tritocerebral commissure. 

 A well-defined ' sympathetic ' system is present. 



The segmented excretory organs consist of antennal glands and not maxillary 

 glands as in the Cypridse. The glands show a typical structure, and there is a well- 

 defined sphincter between end-sac and duct. 



Prof. H. Geaham Cannon. — On an Undamaged Specimen of Nebaliopsis 

 typica G. 0. Sars. 



Nebaliopsis typica was originally described by Sars from a few damaged specimens 

 collected by the Challenger Expedition. Thiele subsequently described the limbs in 

 detail from a crushed specimen collected by the German Deep Sea Expedition. The 

 Swedish Antarctic Expedition of 1898-9 was the first expedition to obtain a complete 

 and undamaged specimen. This was briefly described in a posthumous note by Ohlin, 

 from which it appears that the animal was still alive when it reached the surface. 

 The specimen, however, has been lost, and it is presumed that it went to the bottom 

 with the ' Antarctic' The ' Discovery ' Whaling Expedition collected eight specimens 

 in 1926-7, one of which is complete and undistorted. Photographs of this specimen 

 will be exhibited. 



Prof. F. A. E. Crew. — The Effects of Density on a Mouse Population. 



Reports of Committees. 



Afternoon. 



The Work of the Great Barrier Reef Expedition : — 



Mr. A. P. Orh,.— Conditions in the Sea-water bathing Coral Reefs. 



In temperate waters much information has been gained on the relation between 

 physical and chemical conditions in the sea and the animals and plants which live in 

 it. The larger animals are susceptible to differences in temperature and salinity, 

 while the minute plants, besides being affected by these, depend on traces of various 

 nutritive salts. In temperate waters these nutritive salts show a seasonal cycle, 

 being abundant in winter and disappearing in the surface water when the minute 

 plants remove them in the early spring. Later in the summer they are regenerated 

 again and currents in the sea distribute them throughout the water again. 



In the sea lying between the Barrier Reef and the mainland there is comparatively 

 little seasonal change except in temperature and salinity. The necessary nutrient 

 salts are nearly always present, but invariably in such minute quantities that a large 

 growth of minute plants is not possible. Fluctuations in the degree of oxygenation and 

 in the alkalinity of the sea are thus not marked either. 



In the sea water actually over a coral reef, however, conditions are very different. 

 The intense metabolism going on in coral pools results in enormous fluctuations. 

 The rich animal and plant life causes diurnal variations which are so great that they 

 obscure seasonal variations. The oxgyen content, for example, may vary from less 



