214 CHAPTERS ON EVOLUTION. 



Arctic Seas, which has been shut off from a former marine existence 

 by the conversion of the Baltic fjords or firths into lakes ; geological 

 changes thus inducing alteration in animal species, and " a primi- 

 tively marine animal " thus becoming " completely adapted to fresh- 

 water life." These opossum-shrimps are so called, because the young 

 are carried during development in special sacs or pouches of the parent 

 form. They present in their early history a very interesting connection 

 between the marked change of form in lower crustaceans, and that 

 direct development of the higher forms of which the crayfish is so 

 well-marked an example. Within the egg, Mysis, like the crab, passes 

 through a Nauplius-stage. Thereafter, however, it grows rapidly; and 

 a remarkable circumstance has to be chronicled, namely, that the 

 original skin or integument remains unaltered, and is not moulted, 

 or otherwise made to participate in the succeeding growth of the 

 body. In this feature, as Huxley remarks, the young opossum- 

 shrimp might be justly compared to the pupa or chrysalis of an 

 insect, since it lies, like the latter, within an enveloping skin from 

 which, in due course, the young shrimp emerges. Here, then, the 

 Nauplius-stage is represented as a fleeting period in development ; 

 arid we see in the Mysis, when full grown, a being which has no gills, 



which possesses a large tail 

 or abdomen, and a small body 

 (or head and chest), and which 

 has but rudimentary appen- 

 dages to its tail. Notwith- 

 standing the fact that the 

 development of the Mysis is 

 well-nigh direct, we must not 

 neglect to note the important 

 facts, firstly, that one of its 

 nearest relations (Euphausia) 

 actually leaves the egg as a 

 FIG. 132. PEN^EUS. true Nauplius ; and secondly, 



that the form and figure of the 



adult Mysis itself is perfectly reproduced in the development of the 

 crustaceans of higher type. 



Thus in the lobster, which so nearly resembles the crayfish in 

 its direct development (Fig. 129), and in its imperfectly represented 

 " Nauplius-stage," the young form (named the Zoe'a fiom its 

 analogies to the youthful stage (Fig. 130, a) of the crabs), passes 

 through a Mysis-stage, but thereafter develops into the mature 

 lobster, with well-developed tail-appendages and " head." The 

 idea that in the adult Mysis we may see represented a transitory 

 phase in the evolution of such higher forms as the lobster and 

 crayfish, is a justifiable assumption. It is one, moreover, which 

 appears to be fully proved by the study of the life-history of that 



