566 SECTIONAL TRANSACTIONS.— D. 



Prof. J. Graham Kerr, F.R.S. — Spirula. 



As is well known, this very rare and, so far as its shell is concerned, comparatively 

 archaic Siphonopod has for the first time been obtained in considerable numbers by 

 the Danish ' Dana ' Expedition, and Prof. Johannes Schmidt has for the first time 

 been able to make observations on its behaviour when alive. The investigation of its 

 anatomy has added a considerable amount of detail to what has already been known, 

 and has provided the occasion for the further discussion of various important general 

 problems of Siphonopod morphology. Amongst these is the evolutionary relationship 

 of the three types of chambered shell — straight, exogastrically coiled and endogastrically 

 ooUed. The shell, originally protective, has become hydrostatic in the Siphonopoda. 

 In Nautilus the terminal chamber of the shell is ballasted by its being occupied by the 

 body, while the others are occupied by gas. In the evolutionary series leading up 

 to the present-day Spirula the shell has become mtemal and the terminal chamber 

 greatly reduced, and in this way a condition of unstable equilibrium is induced. 

 In archaic fish with the Polypterus type of lung a similar unstable condition 

 occurs, but this has been corrected in the evolution of modern fishes with their 

 symmetrical air bladder by the air-containing organ having become rotated within 

 the body into a dorsal and stable position. It would appear that the endogastrio 

 position of the shell in Spirula, &c., is due to a similar rotation of the shell, after 

 it has become internal, into the position of stability. The straight type of 

 chambered shell is regarded not as primitive within the group Siphonopoda but as 

 being subsequent to the enclosure of the shell within the body, a further development 

 of the tendency already shown in Spirula, where the spiral curvature practically dis- 

 appears during the later stages of shell formation. 



Prof. R. Broom, F.R.S. — On the Evolution of the Mammalian Vomer. 



Discussion on Bothriocidaris and the Ancestry of Echinoids. (Prof. Th. 

 MoRTENSEN, Dr. F. A. Bather, F.R.S., and others.) 



Afternoon. 



Dr. G. S. Carter. — Lantern Lecture on The Conditions of Life in the 

 Swamps of the Tropics : an Investigation of the Swamps of the 

 Paraguayan Chaco. 



The swamps, which were investigated during a visit to Paraguay in 1926 and 1927 

 by the author and Mr. L. C. Beadle, lie in the eastern part of the Gran Chaco, the flat 

 country stretching from the Rio Paraguay towards the Andes. The part of this 

 country in which the observations were made is in the latitude of the southern tropic 

 and seventy mUes to the west of the Eio Paraguay. The swamps are shallow, contain 

 much vegetation and frequently dry in the cooler months which are here the drier 

 season. Observations of the temperature, pH, bicarbonate-, carbon dioxide-, 

 phosphate- and oxygen-contents of the water were taken continuously during the 

 period October 1926-May 1927 at selected points in the swamps. The penetration of 

 ultra-violet light into the water was also studied. These observations showed that 

 the shortage of free oxygen in the water was very marked and was probably of the 

 conditions studied that of most importance to the life of the fauna. The observations 

 were supported by others made in pools and other waters different in character from 

 the swamps. In these there was never so great a shortage of dissolved oxygen, and 

 at midday the water was often supersaturated with the gas owing to the photosynthesis 

 of phytoplankton. 



The truly aquatic fauna and flora of the swamp were scanty and contrasted in this 

 respect with those of the pools. In the swamp the amount of animal life varied with 

 the amount of free oxygen present but not with the variations of any of the other 

 conditions studied. Many of the forms living in the swamp showed adaptations to 

 life in a medium poor in free oxygen. Especially was this the case among the fishes, 

 many of those occurring in the swamps being adapted to the breathing of atmospheric 

 air. Lepidosiren paradoxa, Symbranchus marmoratus, Eryihrinus unitaeniatus, 

 Calichthys spp., Ancistrus anisitsi, and Hypopomus brevirostris are all adapted to this 



