424 



NATURE 



[June 2, 192 1 



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 

 ta^h^eti, of anonymous communications.] 



^arth-worms, N^ud-worms, and Water-worms. 



I GATHER from various correspondents that the word 

 "earth-worm " requires some elucidation if we are to 

 avoid misconception. I venture to submit the follow- 

 ing statement. The Chaetof>oda, or bristle-footed 

 worms — often called the "Annelids " — form one of the 

 three large branches of the phylum Appendiculata. The 

 Arthropoda ( = Gnathopoda) and the Rotifera are its 

 two other branches. The Chaetopoda are divisible into 

 the class '■ Polychaeta " (all marine) and the class 

 '■ Oligochaeta " — containing very few marine forms, 

 and inhabiting either the slime and mud of fresh- 

 water pools and streams, or the loose, damp " humus " 

 or "earth " of the land-surface. 



I think that the opinion expressed by Mr. Coste (in 

 his valuable letter in Nature of May 19, p. 360), to the 

 effect that " the moist surface " of worms inhabiting the 

 soil '■ must, when underground, frequently or usually be 

 in contact with other moist surfaces, so that the wc«-m 

 is, in effect, partly immersed in water," involves a 

 mistake as to the condition of the earth thus inhabited 

 and the size of the burrow made by the worm. The 

 earth inhabited by earth-worms is not slime or mud, 

 and does not fit closely to the worm's body as would 

 a semi-liquid mud or soft clay. It is, on the con- 

 trary, a loose aggregation^the solid ■ particles of 

 which are largely separated by atmospheric air — and 

 the worm's body does not fit tightly to the walls of 

 its burrow, although a mucous exudation from the 

 worm's surface is often deposited on those walls. An 

 earth-worm in movement is continually changing its 

 dimensions — elongating and becoming narrow, 

 shortening and becoming wider. Hence there is no 

 "immersion " of the worm's body in liquid or semi- 

 liquid material. On the contrary, air has continual 

 access to the worm's surface through the porous soil; 

 and the change in its diameter and its movements 

 must cause the movement of the air in the space be- 

 tween the worm's body and the wall of its burrow. It 

 seems to be necessary to bear in mind the distinctive 

 physical features of the earth or surface soil in which 

 the earth-worms make their burrows — ^as contrasted 

 with either the water or the mud — practically a liquid 

 — in which, other Oligochaeta pass their lives. 



The Oligochaeta are divided into four orders, 

 namely, the Naidiformia, the Saenuridiformia, the 

 Lumbriciformia, and the Hirudiniformia (or leeches). 

 The first of these is a very distinct group in- 

 habiting fresh-waters (only two British species 

 are marine). They are much smaller in size than 

 the others, and are characterised by young forms 

 which reproduce abundantly by bud-fission, and are 

 succeeded by a distinct adult sexual form. The 

 Seenuridiformia and the Lumbriciformia are closely 

 allied — the former being as a rule smaller, more 

 elongate, and more agile in movement than the latter, 

 and less familiar to the inexpert observer. Thev in- 

 habit fresh-waters (a few only are marine) and the 

 mud or slime of fresh-water pools of brackish estuaries 

 and of sluggish streams, whilst the Lumbriciformia 

 are large worms with opaque, thick bodv-wall, which 

 forcibly burrow in loose, air-holding earth, and have, 

 NO. 26q2, VOL. 107] 



as a rule, a close resemblance to our common earth- 

 worm in shape, colour, and habits. The larger 

 Saenuridiformia, such as Lumbriculus and Phreoryctes, 

 have the brown-red colouring of earth-worms, 'and, 

 although differing in shape and movement' from 

 Lumbriciformia, are liable to be mistaken for young- 

 earth-worms when appearing, as they sometimes do irt 

 great number, in the reservoirs and mains of water- 

 supplying companies. 



The Lumbriciformia comprise a great number of 

 genera distinguished by peculiarities of their repro- 

 ductive apparatus, their renal organs (nephridia), and 

 the gizzards and other parts of the digestive canal. 

 They have as a rule a specially rich supply of blood- 

 vessels to the integument which serves as a respiratory 

 organ. This special blood supply is not present in 

 the Saenuridiformia, which have, however, well- 

 developed, deeper-lying vascular trunks holding' haemo. 

 globinous fluid. 



The word "earth-worm" is often applied to the 

 whole group of Lumbriciformia, which are contrasted 

 as "Terricola " with the Saenuridiformia, for which 

 the term "Limicola," or "mud-dwellers," is used. 

 There are good reasons, however, for limiting the 

 word "earth-worm " to the common English earth- 

 worm, Lumbricus terrestris, and the few closely allied 

 species of Lumbricus. Those reasons come to our 

 notice when we are considering the possible drown- 

 ing of the common earth-worm and the respiratory 

 conditions connected with that mishap. They are, 

 first, that a whole family of Sasnuridiform worms is 

 known — the Enchytraeidae — which are not water- 

 dwellers or mud-dwellers (Limicola), but live in humus, 

 and amongst dead leaves, and are, in fact, just as 

 much "Terricola " as are the commoner Lumbrici- 

 formia. And, secondly, there are at least two genera 

 classed with the Lumbriciformia which live, not in 

 the earth, but in open water. One of these is the 

 Criodrilus lacuum (occasionally found in England, but 

 common in Central Europe), which has the appear- 

 ance, size, and inner structure of the Lumbriciformia 

 — and, indeed, is a close ally of Lumbricus ; and the 

 other is the Alma nilotica or Digitibranchus niloticus, 

 which not only lives entirely in the water, but is also 

 provided with a series of filamentous, naked branchiae 

 containing a blood-red vascular fluid. I am not 

 able to state whether Criodrilus has or has not a 

 tegumentary blood supply. It has not, I think, been 

 studied from this point of view. There is no full 

 account (so far as I can ascertain) of the structure 

 of Alma nilotica, nor have illustrations of its anatomy 

 been published, though systematists have given brief 

 accounts of this and allied species. 



I think, then, that it is clear that we must not 

 extend the implications of the word "earth-worm '* 

 when discussing details of structure and physiological 

 adaptation beyond the particular species which has 

 been the actual subject of study. In writing here of 

 the drowning of earth-worms I have intended my 

 statements to apply only to the common British 

 earth-worms called Lumbricus terrestris. Probably 

 they are true of many other Lumbriciformia, but that 

 is only a supposition which must be tested and must 

 not be held as fact until proved by further examina- 

 tion to be so. E. Ray Lankester. 



P.S.— I find that Vejdowski has described a rich 

 network of capillary blood-vessels in the integument 

 of Criodrilus (a Lumbriciform), and that in Limno- 

 drilus Hoffmeisteri (a Saenuridiform) he has found 

 capillaries in small groups of four, ending blindly in 

 the epidermic cell-layer. Such capillaries in the integu- 

 ment are, he says, absent, as a rule, from all Oligo- 

 chaeta except the Lumbriciformia. E. R. L. 



