334 LECTTIRE XV. 



One cannot witness the earlier stages of Branchipus and Apus 

 without being struck by their resemblance to certain forms of Tri- 

 lobites. And so likewise with the larva of Limulus. The argu- 

 ment against the affinity of this genus to the Trilobite which had 

 most weight with Burmeister, was the peculiar bayonet-shaped 

 weapon proceeding from the post-abdominal division of the body in 

 the fully-developed King-crab. Now, when it quits the ovum, this 

 weapon is not developed ; the cephalo-thorax is relatively smaller ; 

 the abdomen longer, and more trilobed, and altogether the larva is 

 much more like the Trilobite than the later stages. The cephalo- 

 thoracic shield is enormous in the larval Sao, but becomes reduced 

 to comparatively small dimensions in the adult animal. Some of 

 the forms of the smaller Trilobites, which figure as distinct genera, 

 e. g. Battus and Agnostus, may also be larval forms of other genera ; 

 for, like the existing Entomostraca, the Trilobites underwent their 

 metamorphoses, which, as in the case of Ogygia, were, also, of a 

 similar nature. Therefore, by these facts in the development of the 

 lower Crustacea — few indeed, I admit, when compared with the great 

 number of known Entomostraca that now exist — a clearer light is 

 thrown on the real nature of those ancient Trilobites than could have 

 been expected in regard to extinct creatures, the affinities of which 

 were so long and so lately considered problematical. 



The ova, after extrusion from the oviducts, are retained and pro- 

 tected in Cymothoa and other sessile-eyed Malacostraca by means 

 of the flabelliform appendages of the thoracic extremities, which ap- 

 pendages are unusually expanded, and ovex-lap each other, so as to 

 form a marsupial cavity or temporary receptacle, in which the incu- 

 bation of the ova is completed. In the Podophthalma, the lamelli- 

 form ciliated appendages of the abdominal segments include similar 

 marsupial or incubatory recesses for the ova. The female lobster 

 and other Macroura are distinguished from the male by the greater 

 development of these appendages ; and in the Brachyura the shorter 

 abdomen or tail is so much more expanded in the females as to cover 

 nearly the whole sternum, and render the sex distinguishable at a 

 glance. 



With regard to the higher group of Crustacea, up to a late period 

 naturalists had believed that they diffijred from insects, as well as 

 from Entomostraca, in undergoing no metamorphosis ; but they offiir 

 diflFerences in this respect in different species. The phenomena of 

 development common to all are the following : — The primary germ- 

 vesicle, after impregnation of the ovum, propagates its progeny not at 

 the expense of the whole germ-yolk, but only of a small portion of 

 it ; so that the process of yolk-fission is a partial, not a total one. 



