December 23. 1920] 



NATURE 



533 



other environmental factors, it may be granted there- 

 tore that growth and reproduction vary m length and 

 periodicity with temperature. Nevertneless, tne fact 

 that the spawning season may be postponed makes 

 us feel unsatisfied with an explanation which transfers 

 in a general way, as, say, by the physical condition 

 of the circulation, the function of intimating to the 

 gonad that it can proceed with its development, 

 licsides, the regularity of the spawning seasons, the 

 manner in which summer and winter are impressed 

 upon the most sedentary animals, the influence of 

 warmth and density in promoting larval life, and 

 other similar considerations, bring us back once more 

 to the question of environment. 



There may be said to be, as is shown in the 

 accompanying diagrams, three phases of life in all 

 animals : immaturity, maturity, and spawning. If 

 the act of spawning is not followed by death, then 

 there succeeds ;i series of similar phases. If we are 

 prepared to recognise that the phase of immaturity is 

 not ended alone by the attainment of the requisite 

 somatic growth, then it is regulated by an organ 

 which acts as an inhibitor of the next phase, and 

 continues to act, not merely until the soma is in the 

 requisite condition, but until the environment is also 

 favourable. Such an organ must be placed in a con- 

 venient situation for receiving impressions from the 

 environment and for communicating to the blood a 

 hormone the function of which is to inhibit the 

 development of the gonad. In the case of aquatic 

 animals we naturally turn to the gills to see if such 

 an organ is provided, and it is already apparent that 

 the thymus has been developed for this purpose. The 

 thymus is a larval organ which develops in associa- 

 tion with the gills, and it is already known that its 

 secretion inhibits the growth of the gonads. .\ 

 thymus-like secretion will probably be found to be 

 at work in this manner in all animals. In the 

 Craniata the thymus is removed from its primitive 

 position in the gills, but it lies near the gills of 

 fishes, and in them and the higher Craniata con- 

 tinues to exercise its function of controlling the period 

 of adolescence. 



When the thymus ceases to supply its secretion to 

 the blood the gonad commences its period of growth, 

 and it is probably then that the thyroid comes into 

 action to control in similar manner the growth of 

 the gonad. The thyroid has an interesting history, 

 and in the higher animals at least its relationship to 

 the gonad has long been known. It is developed in 

 the floor of the gill-chamber, and therefore is in i' 

 favourable position not only for helping in the strain- 

 ing of the fo<Hl bv the gilN, hut also for intimating to 

 the blood the reception of the food, and even other 

 environmental changes. The thyroid, then, probably 

 controls the second phase; and if the thymus, so to 

 speak, during the first is constantly intimating to the 

 gonad that it is not time yet to commence growth, that 

 the environment is not in the rifjht condition, so can 

 we imagine the thvroid controlling the degree of growth 

 until temperature and other conditions are favourable 

 for spawning. Both these ecological organs may 

 be presumed to influence the growth of the .soma 

 according to the environmental changes. The gonad 

 itself during its period of growth contributes messages 

 to the blood which influence growth and produce 

 migrations. 



With these organs we have to associate the pitulfarv 

 bodv, which arises from the ectoderm of the roof jf 

 the mouth, and the parathyroids, which are developed 

 in the gills. The former appears to be very like the 

 thvrnid in function, and the latter are vet llttli" under- 

 stood. Until we know more of these organs it would 

 be idle to speculate as to their primitive functions. 

 The actions of such organs of the environment may 

 MO. 26fio VOT . 106] 



tend to be automatic, as has been suggested, but it 

 is probable that direct impulses are introduced pro- 

 ducing adaptation to changes in salinity and tempera- 

 ture. For example, there are widely distributed 

 species, but restricted to tropical and temperate seas, 

 and others which with little or no modification manage 

 to live in every kind of water from equatorial to 

 Arctic. Many of the species, therefore, which have 

 with great difficulty been differentiated amongst holo- 

 plankton will likely turn out to be more physiological 

 than morphological in character. 



We have yet a lot to learn from biochemical inquiry 

 and from experiment as to these fascinating ductless 

 glands. I have, in the meantime, tried to show that 

 some of them are concerned in conveying impressions 

 of the environment to the soma and to the germ- 

 cells by a series of impulses carried by the blood, and 

 leading to a primitive but effective internal and 

 external integration. It is probable that reproduction 

 in general in Protozoa, and in the cell as in .Metazoa, 

 is so controlled, and that such fundamental effects 

 may have a profound pathological bearing. 



.^LKXANDER MsBK. 



Armstrong College, Newcastle-upon-Tyne, 

 November 8. 



Mode of Feeding and Sex-Phenomena In the Pea-Crab 



{P\nnoiheres pisum). 



The pea-crab which lives inside mussels, cockles, 

 and sometimes even oysters, has been an object of 

 general interest since the days of the Pharaohs, and 

 many legends have been invented of the way in which 

 it feeds and of its relation to its host. Caiman gives 

 an account ("The Life of Crustacea," by W. T. Cai- 

 man, London, 191 1, p. 217) of many amusing habits 

 attributed to this crab by many writers. For instance, 

 the pta-crab has been stated to warn its host of the 

 approach of enemies or of the entrance of prey be- 

 tween its gaping valves, but it will be seen from 

 what follows that the bivalve and crab depicted in 

 Egyptian hieroglyphics to symbolise the dependence of 

 man on his friends is indeed an appropriate symbol. 



A search of modern literature, however, reveals that 

 only vague accounts exist of the manner in which 

 this crab feeds atid lives. Recent work on the mode 

 of feeding in bivalve molluscs and other plankton 

 feeders suggested a possible way in which crabs im- 

 prisoned in these animals might obtain their food. It 

 will be remembered that in those animals which feed 

 on plankton — floating organisms — by using the gill 

 as a food-sieve, the foo<l-material is collected gencrallv 

 into cylindrical masses by means of mucus, and such 

 is the case in mussels, cockles, oysters, and other 

 bivalves. Thus animals living in the unoccupied 

 space between the valves of these bivalves have ready 

 access to a concentrajted supply of food. Now 

 it is easy to watch a pea-crab inside a mussel 

 by cutting a window in the shell of the bivalve 

 and allowing a crab to creep inside it. This 

 has been done, and the bivalve fed on concentrated 

 plankton. It was foun<l that a hungry crab soon 

 b«"gins to feed in the manner described below. 



The larger pea-crabs generallv sit on the iniddle of 

 the bivalve, with a pair of the bivalve's gills on each 

 side. The smaller crabs mav be found in any position 

 inside a mussel, but are generallv somewhere on the 

 gill. When a crab is obser\'ed through n window in 

 a mussel-shell it is seen that such n position is taken 

 up generally so as to bring the edge of n gill-Ienflet 

 up against the abdomen of the crab. Now ns the 

 e<li«e of a glll-leaflet is one of the main food-paths in 

 n bivalve, it will he seen that when the mussel is 

 fee<ling cvlindriral masses of food will ftutomalirally 

 pass along the edge of the gill-leaflets, and thereforif 



