[NOTE:--A critic calls in question the statement on p. 121 respecting the _Amphioxus_. At the outset, however, he admits that in the _Amphioxus_ “the central nervous system and the notochord are not segmented.” In the Annelid, however, the central nervous system is segmented, and there is segmentation of the part which, as a supporting structure, is analogous to the notochord in respect of function--the outer part which represents the exo-skeleton in contrast to the endo-skeleton. He goes on to say that “the gut is not involved [in the segmentation] and exhibits in _Amphioxus_ just as it does in worms differentiations entirely independent of the segmentation of the mesoblast.” Part of this statement is, I think, not congruous with all the facts. In _Protodrilus_, one of the lowest of the _Archiannelida_, “the intestine is moniliform, there being a constriction between each segment” and the next. (Shipley.) Complete segmentation of the intestine is obviously impossible, since, were the canal divided into portions by septa, no food could pass. But the fact that the gut has these successive expansions and constrictions, corresponding to the successive segments, and giving to each segment a partially-separate stomach, shows that segmentation has gone as far as consists with the carrying on of the lives of the segments. No such partial segmentation exists in the _Amphioxus_. Thus, then, three fundamental structures--the directive structure, the supporting structure, and the alimentary structure--are respectively simple in the lowest vertebrate and segmented, or partially segmented, in the lowest Annelid. Again, while it is said that the gill-clefts exhibit segmentation, it is admitted that this has no relevance to any constitutional segmentation: “they are segmented on a plan of their own” irrespective of other organs. Another allegation is that the ovaries of _Amphioxus_ are segmented. Their segmentation, however, like that of the gills, is isolated, and may be considered as illustrating those repetitions of like parts seen in supernumerary vertebræ in various creatures--a repetition which becomes habitual if the resulting structure is advantageous to the species. On the statement that while the _Amphioxus_ has no rudiments of a renal system the Elasmobranch embryo has such rudiments, which are as distinctly segmented as the nephridia of a worm, two comments may be made. The first is that if in these Vertebrates the nephridia bear a relation to the general structure like that which they do in Annelids, then one would expect to find the segmental arrangement shown in the lowest type, as in Annelids, rather than in a type considerably advanced in development. Should it be replied that in the _Amphioxus_ an excretory system had not yet arisen, though one is required for the higher organization of an Elasmobranch, then the answer may be that since the segmental arrangement in the Elasmobranch corresponds with that of the myotomes,