Decembss 1, lUOO.j 



KNOWLEDGE. 



207 



in it, what degree of saliuity, how much light or shade 

 they need, aud what sort of food is suitable to them 

 ht each successive stage. They must be isolated, to 

 prevent confusion with other species, and to protect them 

 from possible enemies. If a,ll the varjMng conditions 

 of their existence were known beforehand, it might still 

 be impossible to reproduce those conditions, but fre- 

 quently they cannot be known until the problem de- 

 manding solution has been already solved. 



Though the isopods are in general at birth nearly 

 like their pai-ents, there is a remar-kable exception 

 supplied by the family of the Gnathiidas. This was for 

 a long time split up into two families, until an ingenious 

 French observer. JI. Eugene Hesse, at last convinced 

 the world, including even his opponents, that the mother 

 and children which had been called Praniza were verily 

 wife and offspring of a husband and father called 

 Gnathia. Sometimes the young of this genus remain 

 parasitic on fishes, gorging themselves, until the time 

 comes for the greater self-restraint attending the remark- 

 able structural changes which discriminate them as 

 males and females. The Cymothoida; also differ, though 

 less strikingly, in the virgin and mature state. 



In other orders many genera have been founded on 

 forms now clearly undei-stood to be immature. Some 

 crustaceans pass through so many distinct stages that 

 a single individual may belong in turn to several of 

 these infantile genera before it becomes a veteran. A 

 crab, for example, will often be a Zoea and then a 

 Megalopa before assuming the features of a Cancer or a 

 Carcinus, or whatever its full grown title may chance 

 to be. In the luminous Eupliauxia seven larval stages 

 demand their several names from Xauplius to Cyrtopia. 

 The slender shrimps of the genus Sergesies have had 

 many an alias, as Acanthosoma, Elaphocdris, Sciacdris, 

 Mastigopus, which remain as a testimony to the trouble 

 they have given zoologists by their fickleness of form 

 before reaching adult life. In this particular genus 

 Dr. H. J. Hansen has by cai'eful examination of a 

 very large collection arrived at a useful result. He 

 finds that in the larvse the eye-stalks are almost always 

 long, the eyes rather or very large, pallid or but 

 partially black, and more or less fungiform in shape, 

 whereas in the adults the eye-stalks are rather short, 

 the eyes smaller and more globular, and totally black. 

 The young have dorsal spines, which disappear, 

 sometimes indeed from the older larva;, but always from 

 the adults. Other characters are available for connect- 

 ing the young of different stages and different species 

 with their proper brothers and sisters and parents. 



The great commercial value of the lobster and the 

 obliging affability with which it so constantly visits the 

 Scandinavian and Anglo-Saxon varieties of mankind 

 have made its life-history a subject of successful study. 

 Quite recently (1898) Mr. J. T. Cunningham has pre- 

 sented a very instructive report to the County Council 

 of Cornwall on the methods and difficulties of lobster- 

 rearing. Dr. Herrick in his valuable book on the 

 .^Vmerican lobster (1895) explains that in the first larval 

 stage, when the animal just escaped from the egg- 

 capsule is only about a third of an inch long, " the 

 body is segmented as in the adult form, the most 

 striking characteristics being the enormous compound 

 eyes, the conspicuous rostral spine, the spatulate telson, 

 and the biramous swimming appendages, which, from 

 their resemblance to the permanent swimming organs 

 of the Schizopods, have given to this and the two suc- 

 ceeding forms the name of ' schizopod larvse.' " There 

 is food for thought, then, in a baby lobster. For 



whereas it is in its earliest life a schizopod, Euphausia, 

 which is still a schizopod when adult, passes as above 

 mentioned through seven simpler stages before reaching 

 maturity. The cnonnous compound eyes are precisely 

 the feature in the juvenile stage of crabs on which the 

 supposed genus Megalopa was founded and named. The 

 spatulate telson occurs also in the larval shrimps. The 

 conspicuous rostral spine is not confined to young 

 lobsters, but is a character which numbers of young 

 malacostracans delight to display. This is very strik- 

 ingly seen in the zoea of our common ForccUana longi- 



Zoea longispina, Dana. Porcellaiiid larva. From Dana. 



cornia (Linn.), and equally so in Dana's Zoea Inngii^pina 

 from the Sooloo Sea. Dana thought that his species 

 might be the young of an Erichthus, it not being known 

 at that period that Erichthiia is itself one of the genera 

 founded on the young of the Squillid;e. It seems 

 probable that the spikes with which larval crustaceans 

 are so abundantly furnished form a defensive armour 

 against foes of approximately their own size. The glassy 

 transparence which many of them share with other 

 animals that frequent the surface of the sea is also 

 likely to be in some measure protective. Whether 

 their large eyes are, as the wolf said to little Red 

 Ridinghood, " the better to see you with, my dear," 

 seems a little uncertain. Either for escaping enemies 

 or for capturing food the small size of these larval forms 

 and their limited powers of locomotion must put most 

 of them much at the mercy of chance, making it rather 

 disagreeable than otherwise for them to have a good look 

 at the unattainable or the inevitable. Perhaps their 

 eyes, by the impression they receive of light and dark- 

 ness, are chiefly adapted to warn them of the depth of 

 water safest for occupation at different parts of tho 

 day. But, however particular points of structure may 

 be explained, the predominant interest lies in the dif- 

 fusion of the same structures among the young of animals 

 which at a later age are almost violently distinguished 

 in size and shape and habits of life. Convergence of 

 characters and the influence of environment seem in- 

 adequate to explain these phenomena, if we exclude 

 the hypothesis that the Crustacea which exhibit them 

 have had a common origin and a slow evolution along 

 many divergent lines from ancestors of very simple 

 structure. 



What is now common knowledge as to the meta^ 

 morphoses of Crustacea, at an earlier part of this 

 nineteenth century was disputed by naturalists of re- 

 markable eminence. As now and then happens in 

 controversy, those who opposed the truth did so nob 

 out of sheer perversity or narrow-mindedness, but on 

 the facts of observation — only not enough facts. We 

 have noticed that a lobster leaves its egg-shell as a 

 schizopod, and that a schizopod proper goes through 

 many preliminary stages which are dispensed with in 

 the hatched lobster. It is conceivable, therefore, that 

 larval stages which are conspicuous in one set of crusta- 

 ceans may be entirely dispensed with in another set. 

 This is what really occurs in some of the land crabs and 

 river crayfishes, and it was from these exceptional 

 forms that the early opponents of crustacean meta- 



