MaV 12, 192 l] 



NATURE 



129 



Letters to the Editor. 



\The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Earthworms Drowned in Puddles. 



I HAVE long been familiar with the frequent occur- 

 rence of dead earthworms in surface "puddles" 

 alongside gravel walks or roads, as described by Mr. 

 Friend in Nature of April 7, p. 172. I have sup- 

 posed that they were "drowned " owing to the 

 amount of free oxygen in the stagnant puddles being 

 insufficient for their respiration. So far as I recollect, 

 earthworms are not drowned (or, at any rate, not 

 quickly) if they get into cool, clear, running water — 

 which, presumably, contains a larger amount of dis- 

 solved free oxygen than does the rain-water accumu- 

 lated about dead leaves and deoxidising or "reducing " 

 mud. (See on this matter Darwin's "Vegetable Mould 

 and Earthworms," pp. 13-16.) I confess that I do not 

 know the facts as to the percentages of free oxygen 

 and of oxygen-seizing matter in natural fresh-waters, 

 or, indeed, in sea-water, in various circumstances ; nor 

 do I know the percentage of free oxygen necessary in 

 water in order that it may — even for the brief period 

 of an hour or two — support the life of an earthworm. 

 I should be glad to know if these quantities have been 

 determined. It is a common practice to kill earth- 

 worms for dissection by drowning them, but I think 

 the water used is warmed. Many years ago I em- 

 ployed " normal saline solution" in the dissecting 

 trough. 



The respiration of the earthworm is carried out 

 through the fine capillaries in the skin, which exposes 

 a moist surface like that of a "lung" to the atmo- 

 sphere. It is abnormal for it to be out of contact 

 with atmospheric oxygen, even in the deepest burrows 

 made by the worm. The abundant haemoglobin in 

 the blood of the earthworm must be kept charged 

 with oxygen by its rapid passage through the ex- 

 tremely delicate capillaries of the skin, separated only 

 from the atmosphere (as is the blood in the capillaries 

 of a lung) by a moist membrane of extreme tenuity. 

 How far this lung-like surface of the earthworm's 

 body can suddenly take on the function of aquatic 

 respiration is a question which some naturalist with 

 a laboratory to work in should determine. 



There are one or two striking facts in this con- 

 nection which deserve consideration. First, there are 

 numerous aquatic "water-breathing" Oligochaeta 

 closely allied to the earthworm, but they are not 

 capable of aerial respiration as an alternative. Some 

 of them inhabit black, foul mud at the bottom of 

 ponds, but, as a rule, they inhabit well-aerated waters. 

 The commonest of them all, Tubifex rivulorum, is 

 extremely sensitive to the lowering of the percentage 

 of dissolved oxygen in the water in which it lives. 

 A handful of some thousands of these worms, if 

 placed (with a little river-mud) in a basin standing 

 on a " sink " under a tap giving a small stream into 

 the basin which overflows into the sink, will group 

 themselves in a definite order, their heads downwards 

 and their tails free and undulating in a constant 

 rhythm, the blood-vessels in the tails thus carrying on 

 active respiratory gas-exchange. They will flourish 

 thus, grow, and reproduce (by eggs) for months ! But 

 if the flow of fresh, oxygen-holding water from the tap 

 is shut off the rhvthmic movement ceases, the worms 

 separate and exhibit spiral contortions. They die in 



NO. 2689, VOL. 107] 



the course of a few hours if the flow of water be not 

 renewed, but when it is they at once recover and re- 

 group themselves. 1 suppose (but have no further evi- 

 dence) that they are as sensitive to the arrest of their 

 normal aquatic respiration by loss of oxygen-carrying 

 water as the earthworm is to the arrest of its normal 

 aerial respiration by submersion. 



On the other hand, it seems that one, at any rate, 

 among our fresh-water worms is fairly tolerant of 

 both the alternative conditions. 



The "medicinal leech" (not to mention other 

 leeches, such as Trocheta viridis and the numerous 

 land-leeches) can live for many days out of water in 

 "moist" surroundings, and also flourishes in sub- 

 mergence. The integument in the leech and the sub- 

 jacent structures are firmer, and yet more elastic, 

 than in the earthworm ; and (as I showed nearly 

 forty years ago) the branches of a very fine network 

 of capillaries containing haemoglobinous oxygen- 

 seeking blood are actually distributed between the 

 individual units of the single layer of cells which forms 

 the epidermis. This brings them even closer to the 

 atmospheric oxygen than in the earthworm. It seems 

 that the leech shows the possibility of the same sur- 

 face acting for either aquatic or aerial respiration. 

 The exchange of the one respiratory medium for the 

 other, without change in the respiratory organ, is 

 exhibited by certain pulmonate Gasteropods allied 

 to Limnaeus, which in the Lake of Geneva inhabit 

 deep water and take water into the lung-cavity. Con- 

 versely, the gill-chamber of some Gasteropods (Cyclo- 

 stoma) becomes converted into a lung, as is also the 

 case in various fishes liable to conditions of drought. 



The presence, and also the absence, of haemoglobin 

 in the blood and in certain tissues of animals have 

 an important relation to the special adjustment of 

 various invertebrate animals to peculiar difficulties 

 and requirements in regard to the supply of oxygen 

 needful for respiration. I cannot in this letter even 

 state the case adequately. For many years, by use 

 of the microspectroscope, I have accumulated facts as 

 to the distribution of haemoglobin, but what is now 

 especially needed is experiment and quantitative 

 measurement to determine what is the significance of 

 the presence of haemoglobin in each case. To cite 

 only a few cases, we ought to ascertain : — 



(i) What exactly is the function of the haemoglobin 

 dissolved in the striped muscular tissue of vertebrates ? 



(2) What is its value in the muscular tissue of the 

 lingual. apparatus of all Gasteropods and Cephalopods, 

 though otherwise absent from those animals? 



(3) What is the explanation of the single exception 

 to the rule as to glossophorous molluscs just stated, 

 namely, the exceptional presence of abundant haemo- 

 globin dissolved in the rich red blood of the flat-coiled 

 pond-snail (Planorbis), although it is absent from the 

 blood of the common pond-snail (Limnaeus) and of all 

 other Gasteropods and Cephalopods ? Again, what is 

 the special value of haemoglobin in the blood (in the 

 form of red blood-corpuscles) of Ceratisolen legumen, 

 whilst it is entirely absent from the common razor- 

 fish (Solenensis) and from every tissue in practically 

 all other Lamellibranchs excepting Area and Pectun- 

 culus, which have (as has Ceratisolen) red hsemo- 

 globinous blood-corpuscles like those of a frog? 



(4) What is the physiological significance of the fact 

 that all Hexapod insects of all kinds are totally devoid 

 of haemoglobin in any of their tissues, excepting the 

 so-called "blood-worm" or larva of the Dipterous 

 midge, Chironomus, in which the blood-fluid ^not 

 corpuscles) is richly coloured by it? 



(5) Similarly, why of all the great tribe of Crus- 

 tacea are the archaic Apus (which has blood as red as 



