Phylogeny of Animals 533 



agreement with the principle that Korschelt-Heider lay down 

 for Crustacea. 



In relation to the Crustacea the eyes of rotifers are worthy 

 of study. In the latter four paired eyes, or two paired and 

 a median eye, or two paired eyes, or as in Anurcea and Notholea 

 one median eye may appear, or as in Noteus they may be want- 

 ing. But Hudson observes regarding Notholea longispina "the 

 adult had a single red eye, at the lowest part of the occiput, 

 near the dorsal surface. Mr. Levick says that many of the 

 first specimens he found had two eyes; and that he thinks 

 these animals were young ones. It would be very curious 

 should it prove to be the case that two eyes in the young ap- 

 proach with age, and coalesce in the adult, especially as Brach- 

 ioni have an eye which has every appearance of being a coal- 

 esced pair" (169, II: 126-27). 



Now in larval Crustacea there is a single median eye that is 

 retained in the simple group of the Copepoda. Each is made 

 up of two lateral and a median eye fused together, since there 

 are three hollow masses of pigment tissue behind retinal tissue, 

 and often three distinct nerves pass to these from the anterior 

 ganglionic region. So it might well be considered that four 

 primitive eyes evolved in successive pairs in at least some roti- 

 fers, that two slowly fused in the middle line, and that later 

 fusion of this mass with the laterals occurred to form an appar- 

 ently single but truly complex eye, with complex nerve supply. 

 The compound eyes of many Crustacea have evidently devel- 

 oped as a character of the group, that is an advance on roti- 

 fer an organization. 



The entire body or a large part of it is, in many crustaceans, 

 surrounded by a common shell, that can only be understood 

 phylogenetically and morphologically by reference to the Roti- 

 fera. This may be "a flat triangular or somewhat oval shell" 

 or "a triangular dorsal shield to start with," or a circular or 

 oval deeply convex lorica. But in the primitive group of 

 branchiopodan Entomostraca the dorsal shell does not appear 

 till after the body and some of its appendages are formed. 

 So in Crustacea as in Rotifera we have a non-loricate and 

 later a loricate stage of advance. In such Entomostraca as 

 Moina a patch of cells forming the nuchal gland appears in 

 similar position to the single clear vesicle already referred to 

 in rotifers. Though various views have been expressed re- 

 garding it, it seems probable that both represent dorsal shell 

 glands. Therefore the embryonic dorsal shell of Crustacea 

 would be homologous to the dorsal shell of loricate rotifers, 

 of brachiopods, and of molluscs. 



