THE DEVELOPMENT OF MYSIS. 303 



slender, and the larva takes on the characters of one of the 

 Anomura. In this stage it has been named Megalopa. By 

 further changes in the same direction, the Anomuran con- 

 dition passes into that of the young Brachyuran. All these 

 modifications of form are accompanied by exuviations of the 

 chitinous cuticula. 



The successive stages are well exemplified by the young 

 of the Shore-crab, Carcinus moenas (Fig. 78, A, B, C). The 

 larva, on leaving the egg, has sessile eyes, a long pointed 

 rostrum, and a spine projecting from the middle of the cara- 

 pace ; rudimentary antennae, and two pairs of locomotive ap- 

 pendages — the rudiments of the anterior maxillipedes. The 

 abdominal somites are without appendages, and the telson is 

 broad and bilobed (Fig. 78, A). 



This, the Zocea-st&ge, after repeated ecdyses, assumes the 

 Megalopa form represented in Fig. 78, B. Finally, the car- 

 apace becomes broader, the abdomen loses its appendages, 

 and is bent up under the thorax ; the peculiarities of the fa- 

 cial region, characteristic of the Brachyura, are developed ; 

 the antennules and ambulatory members acquire their char- 

 acteristic proportions ; and the little Brachyuran by degrees 

 assumes the special peculiarities of Carcinus (Fig. 78, (J). 



The development of the Opossum Shrimp (Mysis) 1 is par- 

 ticularly interesting, as it appears to indicate the relations 

 between the two modes of development, that with and 

 that without metamorphosis, which obtain in the Crustacea 

 (Fig. 79). 



The ova consist of a vitelline mass, inclosed within a deli- 

 cate chorion. The blastoderm appears as an oval patch 

 upon the surface of the yelk (Fig. 79, A, c), thickest in the 

 middle, and here presenting a more or less marked depression 

 (Fig. 79, A, B, c). It is sharply defined from the subjacent 

 yelk (b), and consists of a finely granular mass, in which mul- 

 titudes of nuclei, about -j-gVs- to -g^r of an inch in diameter, 

 are imbedded. 



The blastoderm next becomes larger at one end than at 

 the other, and a median sinuation gradually divides this ex- 

 tremity into two lobes, which will eventually form the ante- 

 rior parietes of the head, and ma} 7 be called the procephalic 

 lobes? 



1 Conf. E. van Beneden, " Developpement des Mysis." ("Bulletin de 

 l'Academie de Bruxelles," 1869.) 



2 It is_ exceedingly interesting to remark the correspondence between the 

 embryonic structure of the head of Mysis (and I may add that of other Arihro- 



