[9] I have not mentioned the Pycnogonidæ, 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 explanations given by Barrande of the development of the latter. According to Mr. Spence Bate “the young of Trilobites are of the Nauplius-form.”) CHAPTER X. ON THE PRINCIPLES OF CLASSIFICATION. Perhaps some one else, more fortunate than myself, may be able, even without Darwin, to find the guiding clue through the confusion of developmental forms, now so totally different in the nearest allies, now so surprisingly similar in members of the most distant groups, which we have just cursorily reviewed. Perhaps a sharper eye may be able, with Agassiz, to make out “the plan established from the beginning by the Creator,”[1] who may have written here, as a Portuguese proverb says “straight in crooked lines.”[2] I cannot but think that we can scarcely speak of a general plan, or typical mode of development of the Crustacea, differentiated according to the separate Sections, Orders, and Families, when, for example, among the Macrura, the River Crayfish leaves the egg in its permanent form; the Lobster with Schizopodal feet; _Palæmon,_ like the Crabs, as a Zoëa; and _Penéus,_ like the Cirripedes, as a Nauplius,—and when, still, within this same sub-order Macrura, _Palinurus, Mysis_ and _Euphausia_ again present different young forms,—when new limbs sometimes sprout forth as free rudiments on the ventral surface, and are sometimes formed beneath the skin which passes smoothly over them, and both modes of development are found in different limbs of the same animal and in the same pair of limbs in different animals,—when in the Podophthalma the limbs of the thorax and abdomen make their appearance sometimes simultaneously, or sometimes the former and sometimes the latter first, and when further in each of the two groups the pairs sometimes all appear together, and sometimes one after the other,—when, among the Hyperina, a simple foot becomes a chela in _Phronima_ and a chela a simple foot in _Brachyscelus,_ etc. And yet, according to the teaching of the school, it is precisely in youth, precisely in the course of development, that the “Type” is mostly openly displayed. But let us hear what the Old School has to tell us as to the significance of developmental history, and its relation to comparative anatomy and systematic zoology. Let two of its most approved masters speak.