414 Dr. r. Miiller on the Darwinian Hypothesis 



abdomen, whicli is divided into several segments, and their tail 

 are destitute of appendages, and the latter is formed of a single 

 piece. Their mandibles are destitute of palpi, like those of in- 

 sects. Their footjaws, of which the third pair are still wanting, 

 have not yet passed into the series of buccal organs, but always 

 present the form of bifurcate natatory feet. There is always a 

 carapace, of which the sides are the seat of the function of 

 respiration. The water, by means of which this function is 

 performed, forms a current which passes beneath the margin of 

 the carapace, and which is produced by the movement of a 

 foliaceous or ligulate appendage of the second jaw. All these 

 larvse have also a pair of large compound eyes, often capable of 

 motion ; and this character, taken together with that of the ca- 

 rapace, which covers the anterior region of the body, enables 

 them to be immediately recognized as young Podophthalma. 



Totally different from these are the larvse of the Ento- 

 mostraca and Cirripedia (including SacculinidfB), which are 

 known under the name of Nauplii. Their oval body is desti- 

 tute of all traces of divisions or segments ; it bears a small, 

 median, frontal eye, and three pairs of natatory feet, of which 

 the first are simple and the others bifurcate. The Nauplii pre- 

 sent no trace of carapace, of paired eyes, or of masticatory 

 organs. 



Lastly, the Edriophthalma present neither the Zoea- nor the 

 iVflMjo/ms-phase, 



The fact that these three groups of Crustacea present essen- 

 tially different modes of development is certainly worthy of 

 remark. Darwin^s theory, by assigning a common ancestor to 

 all these Crustacea, presupposes that this ^ancestor itself pre- 

 sented these different modes of development. This hypothesis 

 undoubtedly appears a bold one; and it has been reserved for 

 Dr. Miiller to demonstrate its truth by the discovery of species 

 with a mixed development, presenting the characters of the dif- 

 ferent groups. 



The most remarkable species in this respect is a Macrurous 

 Crustacean of the genus Peneus, which quits the egg not under 

 the form of a Zo'ea, like the other Decapoda, but under that of 

 a true Nauplius, perfectly similar to those of the Entomostraca. 

 Beneath the skin of this larva the succeeding phase makes its 

 appearance as a little Crustacean with a body divided into seg- 

 ments. Within the first two pairs of natatory feet two pairs 

 of antennae ai-e formed, and vigorous mandibles make their ap- 

 pearance in the third pair. Besides these, new pairs of limbs 

 originate further back. The integuments of the Nauplius are 

 then cast off, and a true Zo'ea issues from it, which can only be 

 distinguished from the Zo'ea of the Alphcei and Palcemones by 



