NOTES. 447 



in large ones. To these belong all the effluvia of decaying matter, sul- 

 phuretted hydrogen, &g. But besides the fact that many insects actually 

 depend on such effluvia for their existence, and many larvse of insects, 

 though air-breathers, live in putrefying matters, we must conclude' that 

 such gases are not universally injurious to all animals alike. Even 

 among the vertebrata the difference in this respect' is considerable. I 

 once kept, in Manila, a large sea-snake at least two feet long, in a glass 

 vessel hermetically closed, and three parts full of water ; the water in a 

 few days became putrid, but the snake lived for twenty-one dajs in the 

 ■ pestilential atmosphere of the vessel containing it. It is even a question 

 whether then death ensued from the direct evil effects of the mephitic 

 vapour, or not merely from the lack of oxygen after the absorption of 

 the small portion contained in the creature's lungs, and in the air enclosed 

 in the glass. 



The reader can also compare the observations of P. Bert, Pliysiologie 

 (lomparee de la Respiration, with those of Milne- Edwards, Leqons d'Ana- 

 tomie et dc Physiologic compa/ree, 



Note S3, page 18i. The view is sometimes put forward that certain 

 stripes running parallel t o the mouth of univalve shells — of the Hehcidfe, 

 for instance — afford an indication as to the age of the aniuoal, each stripe 

 being supposed to correspond to a year's growth (like the annual rings 

 in a tree). This may perhaps —but only perhaps — be true with regard 

 to our northern forms; but even among the land-snails of the Mediter- 

 ranean, it ceases to have any application. I myself saw that in Spain 

 and the Balearic Islands, after a summer's rest of about two months, or 

 even more, almost all the species began to couple, to lay eggs, and to 

 grow again as soon as the autumn rams fell in September. Now, as the 

 eggs of land-snails develope very rapidly, and never remain, like those 

 of many insects, undeveloped through the winter, the young must be 

 hatched out in the autumn ; their growth is probably interrupted during 

 the winter, as is, in fact, not unfrequently indicated by the presence of 

 a stripe. They begin growing again in the spring, and apparently 

 deposit their first eggs before the summer drought comes on ; after their 

 summer rest they lay eggs a second time, but nevertheless continue to 

 grow, and thus form a second line of growth. This, at least, would 

 seem to be the inference from the fact that in the autumn, along with 

 the fully grown specimens, small ones are to be found with only one 

 stripe, and which seem to have been hatched out in the spring. So far 

 as I know, no attention has hitherto been paid to this circumstance. 



Note %i, page 186. Planarian worms are worms of low type and 

 simple structure, for the most part flat, living chiefly in the water ; in 

 the sea they often attain a considerable size, and exhibit the most bril- 

 liant hues. The first discovery of a land Planarian was made by the 

 well-known Danish zoologist, 0. E. Miiller, but his remarks on Playiaria, 

 terregtris excited httle attention till Darwin published his observations 



