as supported by Observations on Crustacea. 415 



the bifurcation of its tail, resembling that of the Copepoda. 

 The compound eyes, indeed, are still wanting, but they soon 

 make their appearance. This Zoevz-phasc afterwards gives place 

 to a phase which can only be designated as the A/yszs-phase, 

 so close is the resemblance of the young Peneus in this stage 

 to a true Schizopod. Finally, a last moult converts this 

 pseu&o-Mysis into a true Peneus. This singular mode of de- 

 velopment is not an isolated fact, for Dr. Miiller has been able 

 to ascertain the occurrence of very similar phenomena of meta- 

 morphosis in several allied species. 



Here, then, we have the development of the Entomostraca con- 

 nected with that of the Podophthalma. The Nauplius-iorm is 

 the simplest under which a Crustacean can quit the egg. The 

 Zoe'tf-phase is a subsequent one. The Entomostraca are hatched 

 in the Nauplius-iovm, and attain their final form before reaching 

 the Zoea -phase. The Podophthalma live for a relatively longer 

 time in the egg, and hence they generally quit it in the Zoea- 

 form without passing through the phase of Nauplius. Some, 

 however, like Peneus, are hatched at an earlier relative period ; 

 and these present the whole normal series of Crustacean deve- 

 lopment, without the omission of a single phase. 



The Edriophthalma (Amphipoda and Isopoda) appear to differ 

 less essentially from the other Crustacea than would seem to be 

 the case from their development, which is very different from 

 that of the Podophthalma and Entomostraca. The Darwinian 

 theory leads to the assumption that their ancestors must have 

 passed through a Nauplius-pha.se or at least a ^oea-phase, 

 although these phases are wanting in the existing species of 

 which we know the development. Dr. Miiller, however, has dis- 

 covered that the Isopoda of the genus Tana'is still retain the 

 characters of incontestable Zoea. Van Beneden had already 

 remarked that Tana'is Dulongii, although a true Isopod, never- 

 theless possesses a carapace like that of a Decapod. This led 

 Dr. Miiller to examine the genus Tana'is, and he soon ascer- 

 tained that these Crustacea, instead of having respiratory 

 abdominal feet like the other Isopoda, have only locomotory feet, 

 into which no blood-globules ever penetrate. To make up for 

 this, respiration is localized in the lateral parts of the carapace, 

 which are constructed for this purpose exactly as in Zoea. The 

 stream of water necessary for respiration is maintained, as in the 

 Zoea and the adult Decapods, by the exognath of the second 

 pair of maxillre, which is deficient in all other Edriophthalma. 



Dr. Miiller records an exceedingly curious fact with regard to 

 a species of the genus Tana'is (T. dubius ?, Kr.), namely, the oc- 

 currence of a new kind of dimorphism in the males. In this 



