REPORT ON THE ISOPODA. 27 



pioment {Serolis necera), or there may be no pigment present {Scrolls (jrac'dls, Serolis 

 hromleyana). 



Development. — Several of the species of Serolis contained eggs within the brood 

 cavity in various stages of development, and although these embryos were not sufficiently 

 well preserved to admit of any examination by means of sections, I have been able to 

 note down some developmental facts which have an important Ijearing upon the affinities 

 of the genus. The development history of Serolis, so far as I have been able to trace it, 

 is strikingly similar to that of the Cymothoadse, which has recently been worked out in 

 detail by Mr. J. F. BuUar.^ The Isopoda generally differ from the Amphipoda in that 

 the embryo has a dorsal flexure in the former group and a ventral flexure in the latter. 

 Thus Fritz Mliller (Facts for Darwin, p. 71) says — "The curvature of the embryo 

 upwards instead of downwards was met with by me as well as by Rathke in Idothea 

 and likewise in Cassidina, Philoscia, Tanais, and the Bopyridse ; indeed I failed to 

 find it in none of the Isopoda examined by me." And at p. 74 — " The Amphipoda 

 are distinguished at an early period in the egg by the difierent position of the embryo, 

 the hinder extremity of which is bent downwards." In the Cymothoadse — at least in 

 those species which are treated of by Bullar in the paper from which the above quotation 

 has been made— the embryo is intermediate between the typical Isopoda on the one hand 

 and the Amphipoda on the other ; it only occupies the ventral surface of the egg, and 

 does not extend so far towards the dorsal surface as in the Isopoda generally, and the 

 telson is bent downwards as in the Amphipoda. In Serolis the embryo is exactly similar, 

 and on PI. X. fig. 7 I have figured a young emluyo of Serolis antarctica; if this 

 figure be compared with the figures of Ci/mothoa in Mr. BuUar's paper, the close similarity 

 between the two will be at once apparent. 



This fact appears to me to be very strong evidence in favour of Milne-Edwards's view 

 concerning the zoological position of the Serolidas, which are regarded by him as a division 

 of the Cymothoadse.^ 



Post-Embryonic Development. — As in many other Isopoda, the males of Serolis when 

 first hatched show none of the distinctive secondary sexual characters of males ; the time 

 at which they assume these characters diflers in difierent species. 



In Serolis cornuta the males are at first exactly like the females in general aspect ; 

 the body is more oval than iu the fully-developed males, being considerably narrower 

 proportionately; the thoracic appendages of the third pair are, as in the females, similar to 

 the succeeding thoracic limbs ; the penial filament of the second abdominal appendages is 

 short, and the sterna of the free abdominal segments resemble those of the female in being 



' Phil. Trans., vol. clxix. p. 50"), 1879. 

 ^ Hist. Nat. d. Cnust., 1840, t. iii. 



