November 3, 1921] 



NATURE 



6^0 



the specimens are dry, and this happens also (prob- 

 ably for the same reason) about a day before the 

 imago escapes ; (4) when the chrysalis is examined 

 with a low power, the gold is seen to be made up of 

 small patches of colour ranging from red to green, 

 with yellow predominating ; and (5) all these colours 

 shift towards the blue as the angle of incidence 

 increases. 



An enlarged sketch from a photograph of a thin 

 section of the wall of the chrysalis is shown in Fig. 2. 

 Photographs of subjects such as these are generally 

 unsatisfactory owing to the impossibility of getting all 

 the significant features in focus at the same time. 



The section shows an inner membrane (a), which 

 stains readily, and is traversed by closely spaced fine 

 lines (about 30,000 per in.), covered by a thin chitinous 

 coat (ft) having many corrugations on the outer side. 



Fu;. 2.- 



-Scction of a portion of a chrysalis of I", urticae. 

 (From a photograph.) 



Of the foregoing observations, which were made on 

 a series of chrvsalids of V. urticae, (i) proves 

 the colour to be the result of interference, and (5) is in 

 agreement with this ; (2) shows that the interference 

 is not a surface effect, but is due to some internal 

 structure in the chitin ; and (2) and (3) together indi- 

 cate that the periodic structure changes its properties 

 and dimensions by wetting and drying. 



In some very thin sections I have thought I could 

 recognise separate layers at the borders of the chitinous 

 coat, but it is difficult to distinguish with certainty 

 th's apparent stratification from the diffraction bands 

 which are often seen at the boundary between two 

 substances of different refractive index. 



A. Maliock. 



Q Baring Crescent, Exeter. 



Sex-manifestations and Motion in Molluscs. 



Ix Nature of October 13, p. 212, Mr. G. C. Robson 

 discusses the application to molluscs of the present 

 writer's suggestion that bisexuality in animals may 

 be a direct physical result of a freely moving habit 

 of life, and that hermaphroditism may be a direct 

 physical result of a sedentary or sluggish mode of 

 life. It was also suggested that it follows from this 

 hypothesis that all sedentary or sluggish animals may 

 be suspected of hermaphroditism where they are now 

 supposed to be bisexual, especially in view of the fact 

 that sex-change may be obscured by a rapid change- 

 over of sex-characters such as we know takes place 

 in the oyster. 



Mr. Robson 's discussion brings out some of the 

 difficulties experienced in applying the generalisation 

 to molluscs, but at the same time illustrates some 

 facts in its favour. It is suggested that the marine 

 Euthvneura, which are hermaphrodite, are as "active" 

 as the Streptoneura, which, except for some sedentarv 

 and parasitic forms, are bisexual. This difficultv is 

 a real one if a comparison be made merelv of the 

 behaviour of the animals in captivitv ; but if 

 ■'activitv " be defined in relation to muscular develof)- 

 ment, it will at once be seen that there is a great 

 Hifferencf between the .Streptoneura and the marine 

 Euthyneura. An .Aplysia. or even a Scaphander, as 



NO. 2714, VOL. I08I 



a type of marine Euthyneura is a flabby and feeble 

 animal in comparison with the strong, powerful, 

 muscular Buccinum as an equivalent type of Strepto- 

 neura. Indeed, the difference in muscularity between 

 the two groups might very well be regarded as of 

 fundamental importance by a biophysicist. 



It is considered undesirable to attempt to define 

 closely such terms as "sluggish " and "active," since 

 if there is any underlying truth in the hypothesis 

 proposed, definitions would soon begin to crj'Stallise 

 out when the problem begins to be seriously attacked. 

 The degree of muscular development will, however, 

 probably enter largely into the definition of " activity' " 

 from this point of view. In this respect the following 

 quotation — warm from the press— -(" Mechanism of 

 Life," by J. Johnstone, 1921, p. 218) is of great in- 

 terest : — "The animaP is characteristically a machine 

 for the conversion of potential chemical energy into 

 movement of body and limbs, and the movement in- 

 evitably leads to friction. " As an illustration of another 

 apparent contradiction of the hypothesis, the Cteno- 

 phora may be mentioned. These forms (excepting the 

 Berce group) are "active " — that is, they move about 

 quickly ; but there are also at least many of them 

 hermaphrodite. The Ctenophores (except the Beroe 

 group), however, move about, not by means of mus- 

 cular movements, but bv the rhythmic action of com- 

 pound flagella, and this, it is submitted, may be 

 regarded from a biophysical point of view as a mode 

 of locomotion fundamentally different from that 

 brought about by muscular action. 



The illustration from Mr. Robson 's letter in favour 

 of the hypothesis relates to the fact that among the 

 Streptoneura, which are tApically bisexual, are a few 

 sedentarv and parasitic forms. Now these parasitic 

 forms are hermaphrodite, and so also are those seden- 

 tarv forms, namelv, Crepidula and Calyptrasa, which 

 have been critically examined. Moreover, it may be 

 observed that such simple cases of hermaphroditism 

 as occur in Crepidula and Calyptraea have been 

 described only recently, and that, therefore, more 

 difficult cases may ver\' easilv have been overlooked ; 

 further, it is not at all incompatible with possible 

 hermaphroditism in a species for a hundred speci- 

 mens of that species (not to mention a mixture of 

 several species) to be examined unsuccessfully for 

 products of both sexes ; even in the oyster Hoek 

 (" Rapport over de oorzaken van den achteruitgang 

 in Hoedanigheid van de Zeeuwsche oester, " p. 174, 

 Uitgegeven door het Ministerie van Waterstaat Handel 

 en Nijverheid, s' Gravenhage, 1902) found only 11 

 hermaphrodite forms out of 130 examined micro- 

 scopicallv, and we now know that sex-change occurs 

 seasonally in this species. In other species — where 

 sex-change mav occur only once in a lifetime — it 

 would be easy to predict a case where less than i oer 

 cent, of a random sample would be hermaphrodite, 

 especially as both sex-products may not be produced 

 simultaneously. 



It is not a difficult matter to raise objections to the 

 general hypothesis in relation to doubtfu'lv active 

 animals ; therefore, results of practical applications 

 of the suggestion are more interesting. For instapr^, 

 are all the sedentar\- Streptoneura, the Scaphooodn. 

 and the Polyplacophora really hermaphrodite, although 

 fhev appear to be bisexual? It must be rememb^^red 

 «^hat Nature can attain the same end in ajl kinds of 

 ingenious ways, as though delighting in obscuring 

 *^he application of anv underlying principles, if such 

 do. indeed, really exist. For example, the present 

 writer applied the hypothesis stated above to an inves- 



^_Prof. Tohnstone is of course referring to the metabolism of undoubtedly 

 a tive animals. 



