[6] Compare the figure given by Darwin (Balanidæ Pl. xxx fig. 5) of the first natatory foot of the pupa of Lepas australis, with that of Lernæodiscus Porcellanæ published in the ‘Archiv für Naturgeschichte’ (1863, Taf. iii, fig. 5). The sole distinction, that in the latter there are only 3 setæ at the end of the outer branch, whilst in the Cirripedia there are 4 on the first and 5 on the following natatory feet, may be due to an error on my part.
[7] Darwin describes as “acoustic orifices” small apertures in the shell of the pupæ of the Cirripedia, which, frequently surrounded by a border, are situated, in Lepas pectinata, upon short, horn-like processes. I feel scarcely any hesitation in regarding the apertures as those of the “shell-glands,” and the horn-like processes as remains of the frontal horns.
[8] The roots of Sacculina purpurea (Fig. 60) which is parasitic upon a small Hermit Crab, are made use of by two parasitic Isopods, namely a Bopyrus and the before mentioned Cryptoniscus planarioides (Fig. 42). These take up their abode beneath the Sacculina and cause it to die away by intercepting the nourishment conveyed by the roots; the roots, however, continue to grow, even without the Sacculina, and frequently attain an extraordinary extension, especially when a Bopyrus obtains its nourishment from them.
[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.
“Whilst comparative anatomy,” said Johannes Müller, in 1844, in his lectures upon this science (and the opinions of my memorable teacher were for many years my own), “whilst comparative anatomy shows us the infinitely multifarious formation of the same organ in the Animal Kingdom, it furnishes us at the same time with the means, by the comparison of these various forms, of recognising the truly essential, the type of these organs, and separating therefrom everything unessential. In this, developmental history serves it as a check or test. Thus, as the idea of development is not that of mere increase of size, but that of progress from what is not yet distinguished, but which potentially contains the distinction in itself, to the actually distinct,—it is clear, that the less an organ is developed, so much the more does it approach the type, and that, during its development, it more and more acquires peculiarities. The types discovered by comparative anatomy and developmental history must therefore agree.”
Then, after Johannes Müller has combated the idea of a graduated scale of animals, and of the passage through several animal grades during development, he continues:—“What is true in this idea is, that every embryo at first bears only the type of its section, from which the type of the Class, Order, etc., is only afterwards developed.”