DEVELOPMENT OF LIMULUS POLYPHEMUS. 197 



comprehensive or synthetic type of all, the Nebaliadse, presenting certain features found 

 only in the Decapoda. It seems impossible to my mind that at the dawn of Silurian life 

 these well marked groups were due entirely to the extinction of multitudes of connecting 

 links, such as Mr. Darwin assumes to have been evolved on the principle of natural selec- 

 tion, with the subordinate agency of sexual selection and mimicry, etc. The groups are 

 almost as clearly marked as in the present time, and such a theory seems to me inadequate 

 to account for the rise of such distinct forms, appai'ently simultaneous in their appearance 

 at the beginning of the Silurian. The forms are remarkably isolated and present every ap- 

 pearance of having been in a degree suddenly produced ; by this I mean that the Archicaris 

 may have evolved the Protocyclus after thousands rather than millions of generations, and 

 that the young differed from the parents in a more or less marked degree, the changes 

 being perhaps superinduced in the em))ryo by change of habits and temperature, and depth of 

 water experienced by the adult. I am strongly inclined to believe that the great differences 

 in the egg, the development of the primitive disk, the singular amnion and the almost ametab- 

 olous young of Liraulus, so unlike other Crustacea, are possibly due to the eggs being laid by 

 the parents in the sand or mud between tides ; this being an exception to all other Crustacea so 

 far as I am aware, unless we except Squilla and Gecarcinus, which also lay their eggs in the 

 sea, but in what manner we are not fully informed. To withstand the action of the waves, 

 the alternations of heat and cold, the chances of being even left for weeks and perhaps 

 months on dry land, before an extra high tide can bear them back into the water, the yolk 

 cells must be small find exceedingly numerous, so as to make the egg contents unusually 

 compact, as they are in Limulus. So also must the chorion be dense and thick and remark- 

 ably unyielding, as we find to be actually the case in the King Crab. Then again, when the 

 embryo increases in size, instead of the young hatching as in other Crustacea, it is in the 

 present instance retained within the egg by a vicarious chorion, i. e. the remarkable amnion, 

 with its toughness and enormous cells, so unique in the Arthropoda. 



Finally, we find that all these characteristics, so unmistakeably correlated with the preca- 

 rious life of the egg, seem to tend to prolong the life of the embryo within the egg, until the 

 young Limulus is hatched in the form of the adult, the metamorphosis being of the slightest 

 possible character. It is evident that the retention of the embryo within the egg, has 

 taken place in order that the embryo may pass through the nauplian and subzoeal condi- 

 tions to attain the larval form so like the adult ; or in other words the metamorphoses aie 

 undergone within the egg instead of after leaving it. That the differences in the egg of 

 Limulus from those of most other Crustacea are due to physical causes, and induced by 

 them rather than correlated with them, is shown in the summer and winter eggs of the 

 Cladocera, the long period of winter causing the eggs which are laid in the autumn to have 

 an extra protection. The eggs of the Estheriadae and of Apus and the Branchipodidse 

 which fall into the bottom of pools which dry up, are also covered by a dense chorion to 

 protect the germ from sudden alternations of heat and cold and wet and dryness ; and in 

 Apus, though the development of the young is accelerated, and the metamorphoses are 

 extra-ovular, we have the amnion acting as a vicarious chorion. On the other hand, the 

 eggs of Nebalia, retained beneath the body until the young are hatched, are soft, with 

 large yolk cells, and a thin, transparent chorion. Here again, in conditions always remain- 

 ing the same, the young is born a zoea, closely resembling the parent, the metamorphoses 

 being intra-ovular. These divergences in the habits and mode either of oviposition or non- 



MEMOme BOST. SOC. NAT. HIST. VOL. II. 50 



