96 HISTOEY OF CRUSTACEA. CHAP. IX. 



responding to the ventral surface of the embryo, a primi- 

 tive band. We now know that in the Copepoda (Clans), 

 in the Ehizocepbala (fig. 64), and, as I can add, in the 

 Cirripedia (figs. 61-63) the segmentation is complete, and 

 the embryos are sketched out in their complete form 

 without any preceding primitive band. Probably the 

 latter will always be the case where the young are 

 hatched as true Nauplii (and not merely with a Nau- 

 plius-skin, as in AchtTieres). The two modes of deve- 

 lopment may occur in very closely allied animals, as 

 is proved by Achtheres among the Copepoda. 17 



17 I have not mentioned the Pycnogonidae, because I do not regard 

 them as Crustacea ; nor the Xiphosura and Trilobites, because, having 

 never investigated them myself, I knew too little about them, and 

 especially because I am unacquainted with the details of the explana- 

 tions given by Barrande of the development of the latter. According 

 to Mr. Spence Bate " the young of Trilobites are of the Nauplius- 

 form." 



