216 BRITISH FOSSIL CRUSTACEA. 



carcinologists have shown, to bring together many of the aberrant members of the great 

 Crustacean class, which are hopelessly isolated from one another in the adult state, but 

 are at once recognisable in their larval dress. 



The formation of the blastoderm accords with that of Gammarus, Asellus, &c, and 

 also, according to Prof. Owen, with Arachnids, the yolk not undergoing segmentation. 



§ 7. Formation of the embryo. — " Between one and two weeks after the eggs are laid 

 {impregnated?} the primitive disk appears, bearing six pairs of appendages (see woodcut, 

 fig. 77). The appendages become more prominent and in a day or two more appear as 

 distinct tubercles." (See PI. XXXIII, fig. 1.) 



In the succeeding stage of development figured both by Drs. Dohrn and Packard 

 (see PL XXXIII, fig. 2, after Packard), the oval body of the embryo has increased in 

 size ; the separation into cephalic and thoracico-abdominal divisions has taken place ; the 

 mouth-opening is seen, and the rudimentary six pairs of appendages are readily made 

 out. The first pair are well marked by their position in front of the mouth and their 

 minute size ; in like manner the sixth pair are distinguished by their greater develop- 

 ment and modified form. Dr. Dohrn also detects in this stage the under lip ■} but his 

 drawing does not show it very clearly. This is followed by the rudiments of the first 

 thoracic plate bearing in the adult the ovaries, and marked in Dr. Dohrn's figure of the 

 embryo by two pores. Behind these come the rudiments of the first two pairs of bran- 

 chiferous plates not yet outspread. The abdomen is then, as afterwards, destitute of 

 appendages. The ' arthomeres,' or body-rings, are also indicated beyond the areola, the 

 sutures extending upwards but not reaching the dorsal side of the egg. These changes 

 occur sometime before the splitting apart of the chorion. The cephalic feet have increased 

 in length and are folded upon themselves. To this period of development Dr. Packard 

 has given the name of the subzoeal stage. 



In another egg (PI. XXXIII, figs. 3 and 4), the chorion of which had not yet split 

 apart, the six cephalic and nine thoracico-abdominal segments are well seen, the sutures 

 forming deep depressions in the yolk, which is pale green as before, and lobulated just 

 within the integument. The bases of the limbs are nearer together, and the line 

 between the cephalic and thoracico-abdominal segments may be distinctly seen, the 

 posterior edge of the former projecting out sensibly beyond the sides of the abdomen. 

 The two basal pairs of abdominal feet (h g) have become more prominent, being thinner 

 and more lamelliform, the third pair {%) being still minute. 



Referring to the early stages of the embryo of Limulus (Woodcut, fig. 77, and 



Mysis, are provided with a marsupium, in which the larvae undergo their metamorphosis ; and the offspring 

 often pass a considerahle time after attaining the perfect state. In these orders we find the young issue 

 from the egg in a far more helpless and immature condition than in Limulus and some other Crustacea 

 whose eggs are abandoned by the parents, reminding one very forcibly of the similar disparity of develop- 

 ment in the young of Insessorial as compared with Rasorial and Natatorial birds. 

 1 "Metastoma," H. W., "chilarium," Owen. 



