38i 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



structures, which are sometimes, as in Hyalaea, 

 inclosed within a special chamber, but in others, 

 such as Clio, are apparently unprotected, and of 

 indistinct nature. 



A very large " brain " — or at any rate a mass 

 of nervous matter corresponding in function to 

 the great nerve-centre of higher animals — is devel- 

 oped in the sea-butterflies, and can be discerned 

 lying beneath the throat, and forming, in fact, a 

 kind of internal collar around the gullet. And 

 nerves accordingly radiate throughout the body 

 from this central mass, and supply the various 

 parts of the organism with feeling and vital power. 

 Especially, as we might expect, do we find the 

 delicate tentacles of the head to receive a large 

 nerve-supply ; and we may also note the presence 

 of two eyes, situated on the back of the neck. 

 These latter organs are not of a very high order 

 of development, but doubtless subserve the func- 

 tion of guiding their possessors in their marine 

 flights. 



It is very curious to observe that, in the course 

 of their development, the members of the higher 

 class of the Gasteropoda already alluded to, at one 

 period evince a strange likeness to the form of 

 our sea-butterflies. The young whelks and their 

 allies first appear on the stage of life as little free- 

 swimming bodies, which move through the waters, 

 each by means of a pair of wing like lobes which 

 spring from the sides of the head. Observing 

 such a form, we cannot but be struck with its 

 close resemblance to the mature form of our sea- 

 butterflies — a resemblance which is, however, 

 wholly lost as the young gasteropod advances fur- 

 ther in its development to attain its adult stage. 



The food of our sea-butterflies appears to con- 

 sist of the more minute marine Crustacea, which, 

 with themselves, haunt the surface of the sea. 



Thus these small beings exist on organisms of 

 still lesser magnitude. But in turn the sea-but- 

 terflies form a large proportion of the food of 

 the largest of animals — the whales themselves. 

 Drawn in myriads into the capacious mouth of 

 the Greenland whale, with the floods of water 

 which the great monster of the deep from time 

 to time imbibes, the sea-butterflies remain en- 

 tangled in the "baleen" or whalebone plates of 

 the jaws, and are thereafter swallowed as nutri- 

 ment ; and the species Clio borealis, from this 

 latter circumstance, becomes known to us under 

 the popular name of " whales' food." Sea-birds 

 also prey upon the butterflies of the ocean, which 

 thus contribute largely to the support of much 

 higher forms than themselves. In the Mediter- 

 ranean Sea, on the Australian coasts, and in the 

 Atlantic Ocean, the sea-butterflies also occur, but 

 not in such numbers as in the far north, whither, 

 to the very home of the pteropods, British enter- 

 prise has advanced on a noble mission of dis- 

 covery. 



Small as are all the existing representatives of 

 the sea-butterflies, it may prove interesting to 

 note in the last place that, in past epochs of this 

 world's history, several relatively gigantic mem- 

 bers of this class appear to have been developed. 

 In some of the oldest (Silurian) rocks, large shells 

 of Pteropoda are discovered as fossils ; one ex- 

 tinct species, known as Conularia, attaining a 

 length of about a foot, and a breadth of fully an 

 inch — dimensions these, of giant kind, as com- 

 pared with the shells of living sea-butterflies. 

 And in more recent rocks, the small, delicate 

 shells of our living Cleodorse and Hyalaea may 

 be found in a fossil state; proving thus to us the 

 ancient ancestry of the existing " butterflies of 

 the sea." — Chambers's Journal. 



Ground-Water and Zymotic Disease. — Mr. 

 Baldwin Latham, in a paper on the sanitary 

 importance of hydro-geological surveys, cites his 

 own observations at Croydon to show the rela- 

 tions between epidemics of zymotic disease and 

 the state of the level of the ground-water. There 

 were sixty-nine houses, some with their cess- 

 pools lying to the north and others with their 

 cesspools lying south of them ; in both, the wells 

 were on the opposite side of the house from the 

 cesspools. In the former houses there had :il- 

 ways been entire immunity from zymotic disease, 



while in the latter the tenants had at different 

 times suffered from this class of diseases. Care- 

 ful investigation showed that the current of un- 

 derground water in that locality was from south- 

 east to northwest. Of course, in the cases where 

 this current first passed by or through the cess- 

 pool and then reached the well, the latter was 

 contaminated ; and from the direction of the cur- 

 rent it is plain that this condition was realized 

 in the case of those houses which had their 

 cesspools on the south and their wells on the 

 north. 



