HARDWICKE'S SCIENCE-GOSSIP. 



105 



air-chamber, the latter formed by a folding of the 

 "mantle." Within this chamber the pulmonary 

 vessels are spread out, and thus the blood is brought 

 into contact with the oxygen of the contained air. 

 The air, of course, soon becomes vitiated by this 

 process, and our snail has to seek the surface of the 

 water and open the air-chamber, which becomes 

 charged immediately with pure air. The blood thus 

 purified returns to the heart, whence arteries discharge 

 it to various parts of the body ; but although there is 

 a well-defined auricle and ventricle there is no system 

 of vessels, beyond mere channels and folds of the 

 various portions of the body. Thus the pale bluish- 

 white or colourless blood circulates, without any 

 definite course, about the general body-cavity, and 

 around and within the various organs there contained. 

 We have briefly mentioned the nervous system ; let 

 us see more particularly of what this consists. Our 

 water-snail, in common with all other snails, has no 

 true brain. Its chief centres of nerve force are known 

 as "ganglia," and are six in number. These are 

 known as the cerebral, the pedal, and the parieto- 

 splanchnic, of each of which there is a pair. First, 

 the cerebral ganglia. These will be found one on 

 each side of the gullet, just below the buccal-mass, 

 connected by a cord across the gullet. From each 

 of these two cerebral ganglia, or nerve centres, minor 

 nerves are given off to the organs of the mouth, the 

 tentacles (popularly termed "horns") the eyes, and 

 the buccal-mass. Next we have the pedal ganglia, 

 which each give off nerves to the muscles of the foot, 

 and to the "otoliths," of which latter more anon. 

 The pedal ganglia are each attached to the cerebral 

 ganglia by a long cord or "commissure," and by a 

 similar connection to the parieto-sislanchnic ganglia. 

 The latter have a somewhat imposing name, the 

 plain English of which signifies that they are the 

 nerve-centres for the intestines and the posterior 

 portions of the animal. As we have stated they 

 are connected with the pedal ganglia, but there 

 is also direct communication with the cerebral 

 ganglia by another long commissure. They give 

 off nerves to the sides of the body, the shell-muscle, 

 the air-chamber, the heart and great vessels. 

 {To be continued.') 



The Cuckoo. — With respect to a cuckoo taken 

 at Westbourne on the 21st of June, 1880, two circum- 

 stances seem worthy of note. Its plumage still 

 (March, 1882) retains much of the colours assigned by 

 Jenyns to the "young of the year." It has several 

 brown feathers on the head, and the wings exhibit 

 scarcely any change from the immature plumage. 

 Another point is that since last Michaelmas it has 

 drunk no water and refuses it when offered. As, 

 unlike parrots, it has had no bread and milk, this is 

 the more observable. Can any one inform me if this 

 is a usual habit of the cuckoo ? — F. If. Arnold. 



OBSERVATIONS ON THE INFLUENCE OF 

 THE SUN'S RAYS ON THE DEVELOP- 

 MENT OF THE OVA AND TADPOLES OF 

 THE FROG. 



By Alexander M. McAldowie, M.D. 



SOME years ago, while investigating the colour- 

 ing matter of animals, the presence of a uniform 

 layer of dark pigment covering the ova of the frog 

 attracted my notice, as it is peculiar to the eggs of 



^i 



Fig. 73- 



Fig. 74. — Pigment-cells of Tadpole of Newt. 



Fig. 75. — Frog-spawn. 



the tailless batrachians. It appeared to me to be 

 probable that this layer of pigment must have refer- 

 ence to the action of the sun's rays, and 1 made the 

 following experiments to discover, if possible, whether 

 the process of hatching is affected by the presence 

 or the absence of light. But first let me describe 



