CHAP. XII. EVOLUTION IN CRUSTACEA. 125 



particularly instructive. Almost all the peculiarities 

 by which they depart from the primitive form of the 

 Zoea of Peneus (figs. 29, 30, 32), may in fact be con- 

 ceived as transferred back from a later period into this 

 early period of life. This is the case with the large 

 compound eyes, with the structure of the heart, with 

 the raptorial feet in Squilla, and with the powerful, 

 muscular, straightly-extended abdomen in Palcemon, 

 Alplieus, Hippofyte, and the Hermit Crabs. (In the 

 latter, indeed, the abdomen of the adult animal is a 

 shapeless sac filled with the liver and generative organs, 

 but it is still tolerably powerful in the Glaucothoe-st&ge, 

 and was certainly still more powerful when this stage 

 was still the permanent form of the animal.) It is also 

 the case with the abdomen of the Zoea? of the Crabs, 

 the Porcettance, and the Tatuira, which is still powerful, 

 although usually bent under the breast ; the two last 

 swim tolerably by means of the abdomen, even when 

 adult, as do the true Crabs in the young state known as 

 Megalops. It is the case, lastly, with the conversion of 

 the two anterior pairs of limbs into antenna?. The 

 second pair of antennae, which, in the various Zoe'a? 

 always remains a step behind that of the adult animal, 

 is particularly remarkable. In the Crabs the " scale " 

 is entirely wanting ; their Zoeae have it indicated in the 

 form of a moveable appendage, which is often exceed- 

 ingly minute. In the Hermit Crabs a similar, usually 

 moveable, spiniform process occurs as the remains of 

 the scale ; their Zoeae have a well-developed but in- 

 articulate scale. A precisely similar scale is possessed 



