534 



NATURE 



[December 23, 1920 



along the part of the gill to which the crab has 

 approximated its abdomen and within easy reach of 

 its claws. When feeding the crab moves, or rather 

 scrapes, its daws over the gill, and by means of the 

 hairy nature of its claws easily catches hold of the 

 mucus-strings entangling the food and transfers them 

 to the region of its mouth. The claw is now scraped 

 over the region of the mouth, and the specially hairy 

 mouth-parts of the crab, working in an opposite 

 direction, comb off the strings of food and press them 

 into the mouth. The claw of this crab is apparently 

 specially modified to enable it to be flexed about the 

 middle of its length for giving the unusual scope of 

 action required for the scraping movements. 



It is thus seen that the pea-crab is a parasite, and 

 no advantage of its presence to the host has yet been 

 detected, unless it be the occasional association of 

 the palps with the crab, for in this case the crab 

 would be relieving the host of the necessity for trans- 

 porting and expelling undesirable food ; on the other 

 hand, the' crab would appear to do no more harm to 

 the mussel than to make it collect food faster or for 

 a longer time than would be necessary in an abun- 

 dant supplv of food if the crab were not there. 



There is thus good reason to believe that those 

 allies of the pea-crab which are known to live in 

 plankton-feeding animals — for example, other bivalves 

 and Ascidians — will be found to feed in the same way 

 as this crab. 



In the search for these pea-crabs during the summer 

 it was observed that berried females were frequently 

 found alone in a mussel, and that males appeared to 

 be scarce ; the method of fertilisation, therefore, was 

 not understood, and appeared to be worth investi- 

 gating. It is well known that in this crab there is 

 a marked sexual dimorphism, the males being small, 

 verv hairv, and, it may be added, very active, 

 while the females are large, relatively smooth, and 

 extremely passive. Moreover, tiny females have 

 apparently never been described. There was thus the 

 additional problem of interest of the whereabouts of 

 the tiny females— if, indeed, tiny females should 

 exist. The whole of the phenomena suggested pro- 

 tandry — a suggestion that was strengthened by my 

 obtaining a tiny female form moulted from a minute 

 supposed male, crab with a carapace only 27 mm. 

 wide. This tiny female form — now 33 mm. in dia- 

 meter — however, was found to have enormous 

 spermothecae crammed full of mature spiked sperm 

 and onlv a trace of gonad, which was, however, 

 obviously female. The moult of this sjsecimen was 

 now carefully examined, and found to possess the 

 full number of abdominal appendages characteristic 

 of the female, although otherwise not recognisable 

 externally from a male. 



Manv other male-like tiny female specimens have 

 been obtained, and also a series of moults which leave 

 no doubt that the young male-like female attains the 

 adult characters through a succession of moults. It 

 is an interesting fact that all the larger females 

 examined have been found to have their spermothecae 

 always full of mature sperm. It would therefore 

 appear that this pea-crab copuljites precociously at an 

 extremely early age before settling down to its later 

 sedentary life, and, so far as my observations have 

 gone, it would seem that no further meeting with a 

 male is necessary, although males have been found 

 in the autumn inside the same host as large females ; 

 a newlv moulted female, however, appears to have 

 no charm for a male. Sexual dimorphism in the 

 species is thus seen to be accompanied by dimorphism 

 of the females, and the change from the male-like 

 form of the female to the adolescent young-bearing 

 female form appears undoubtedly to depend upon, and I 

 to occur after, copulation. It would also appear that 

 NO. 2669, VOL. 106] 



copulation normally takes place inside the host, and 

 that the males visit mussels in their search for 

 females, since unwary male crabs have been found 

 with their legs or bodies trapped bv the mussel closing 

 its shell before the crab could get inside. These crabs 

 survive the rough treatment by reason of their extra- 

 ordinarily strong carapace, and creep inside the mussel 

 later when it must perforce relax and open its shell 

 in order to breathe. The male-like female has a 

 similarly hard carapace which prevents the animal 

 being crushed to death if unluckily trapped by the 

 mussel destined to become a host. Individual Vrabs 

 have been found to be lacking a leg which might 

 very well haye been lost in this dangerous operation. 



The life-story of this animal is probably not yet 

 complete, since the tiny male-like female appears to 

 possess a different sexual apparatus from the older 

 forms, and a single male form has been found with 

 a carapace closely resembling that of a female. It 

 is hoped, therefore, to complete the investigations in 

 the near future. J. H. Orton. 



The Laboratorv, Citadel Hill, Plymouth, 

 November 10. 



The Energy of Cyclones. 



Mv objection to .\lr. Detley's suggestion (Nature, 

 December 16, p. 502) that cyclones are caused by an 

 access of warmth in the stratosphere is that in that 

 case the troposphere ought to bulge upwards over the 

 cyclone, whereas, in fact, it bulges downwards. The 

 inversion at the boundary renders it certain that the 

 interchange of air between the troposphere and the 

 stratosphere is very slow, and for the few days of the 

 life of a cyclone we inay, to get a clear idea, imagine 

 a light but impermeable film to exist at the boundary. 

 If a patch of air above the film were warmed by any 

 means it would expand outwards and reduce the 

 weight on the film, which should, in consequence, rise. 

 If, on the other hand, the film were drawn down- 

 wards, the temperature of the air above it would rise, 

 because each air-particle as it lost in height would 

 come under an increased pressure, and be warmed 

 adiabatically. If, then, the air is drawn outwards hori- 

 zontally from a cyclone in the upper part of the tropo- 

 sphere, the conditions as to temperature and the posi- 

 tion of the top of the troposphere that are known to 

 prevail are readily explained. 



The point mentioned by Sir Oliver Lodge in his 

 letter in Nature of November 25 has been, I think, 

 put forward by von Besold and others, but Sir Oliver 

 seems to have overlooked the result of the heat set 

 free by the condensation of the vapour. Could a 

 cubic metre of damp air be confined in an adiabatic 

 but extensible balloon and the vapour be condensed by 

 any means, the result would be an increase of volume, 

 for the expansion due to the heat produced by the 

 condensation would far more than balance the con- 

 traction due to loss of pressure. If, indeed, the heat- 

 energy due to the latent heat of vapour all took the 

 form of kinetic energy in the atmosphere, quite a 

 trifling rainfall would suffice to produce over the same 

 area the most violent cvclone ever recorded. 



W. H. Dines. 



Benson, Wallingford, Berks. 



The Mechanics of Solidity. 



In Nature of November 18 last Mr. J. Innes 

 pointed out that if a number of substances be 

 arranged in order of increasing coefficient of 

 linear expansion, then they are very nearly in a 

 series of decreasing hardness, as shown by Brinell's, 

 Moh's, or Auerbach's test,' and he has,' therefore, 

 suggested, that it should be possible to obtain a fair 



