M. A. Giard on the Genus Entoniscus. 153 



said that wc find ourselves in a state of actual poverty, since 

 all these ova are at the same degree of development, and it is 

 impossible to make them continue their evolution outside of 

 the maternal organism. 



I can say scarcely any thing about the embryo before its 

 escape from the egg. The segmentation appears to be holo- 

 blastic ; the embryo is bent backward, like that of all the 

 Bopyrida3. The first six pairs of thoracic feet appear at first 

 all similar ; the seventh segment is destitute of appendages. 



The five pairs of abdominal feet, which, in my opinion, 

 correspond to the natatory feet of the Cypris form of the 

 Cirripedes, or to the cirri of the adult Lejxis, appear first, and 

 all together. 



On each side of the embryo, at the stage represented by 

 fig. 9, we see a line of refringent bodies. I have seen similar 

 lines in the embryos of several genera of Bopyridse. In En- 

 toniscus we see later on at the same place (fig. 9) two lines of 

 pigment-cells. The pigment of Entoniscus has never offered 

 me the characteristic odour of that of the Cryptonisci, an 

 odour which is correctly indicated by Dr. P. Fraisse. 



The embryo at the moment of its escape from the egg 

 (fig. 10) is about 0*3 millim. in length. It presents two 

 pairs of antennas : the inner ones, which are short, are termi- 

 nated by two tufts of setas ; the outer ones, which are much 

 longer, are formed of six joints, of which the third bears two 

 setas, much longer than the others. The front is nearly 

 straight, as in the embryo of Entoniscus porceUance. Besides 

 the lateral eyes, which are double and correspond to the 

 definitive eyes of the ordinary Isopods, it possesses a median 

 eye, presenting exactly the structure of the nauplian eye of 

 the Copepods &c. We find in it, in fact, two crystallines 

 (figs. 11, 12), two optic nerves, and a strong black pigment- 

 spot, the anvil-like form of which perfectly recalls that of the 

 eye of the nauplius of the Cirripedes or of the free Cope- 

 pods. 



Fritz Miiller indicates in the middle of the front of the 

 embryo of E. porceUance a transparent spot, which, no doubt, 

 is only the rudiment of a similar nauplian eye. 



Dr. Fraisse has also observed something analogous in a 

 species of Cryptoniscus (C monophthalmus, Fr.). The male 

 of this species possesses a single median eye instead of the 

 lateral eyes of the other types of the same genus*. 



* The Ci/pris-lavva of an undetermined Cirripede, taken at Wimereux 

 in September in the muslin net, also presented thru' eyes— the median 

 eye ol the nauplius and the two ordinary lateral eyea of the pupa-stage. 

 A median pigment-spot also exists, besides the lateral eyes, in a branchio- 

 pod crustacean, Holopedium t/ibberum, Zaddach. 



