(541) J. Kaufmann. “Ueber die Entwicklung u. systematische Stellung d. Tardigraden.” Zeit. f. wiss. Zool., Bd. III. 1851.
Summary of Arthropodan Development.
The numerous characters common to the whole of the Arthropoda led naturalists to unite them in a common phylum, but the later researches on the genealogy of the Tracheata and Crustacea tend to throw doubts on this conclusion, while there is not as yet sufficient evidence to assign with certainty a definite position in either of these classes to the smaller groups described in the present chapter. There seems to be but little doubt that the Tracheata are descended from a terrestrial Annelidan type related to Peripatus. The affinities of Peripatus to the Tracheata are, as pointed out in a previous chapter (p. [386]), very clear, while at the same time it is not possible to regard Peripatus simply as a degraded Tracheate, owing to the fact that it is provided with such distinctly Annelidan organs as nephridia, and that its geographical distribution shews it to be a very ancient form.
The Crustacea on the other hand are clearly descended from a Phyllopod-like ancestor, which can be in no way related to Peripatus.
The somewhat unexpected conclusion that the Arthropoda have a double phylum is on the whole borne out by the anatomy of the two groups. Without attempting to prove this in detail, it may be pointed out that the Crustacean appendages are typically biramous, while those of the Tracheata are never at any stage of development biramous[213]; and the similarity between the appendages of some of the higher Crustacea and those of many Tracheata is an adaptive one, and could in no case be used as an argument for the affinity of the two groups.
The similarity of many organs is to be explained by both groups being descendants of Annelidan ancestors. The similarity of the compound eye in the two groups cannot however be explained in this way, and is one of the greatest difficulties of the above view. It is moreover remarkable that the eye of Peripatus[214] is formed on a different type to either the single or compound eyes of most Arthropoda.
The conclusion that the Crustacea and Tracheata belong to two distinct phyla is confirmed by a consideration of their development. They have no doubt in common a centrolecithal segmentation, but, as already insisted on, the segmentation is no safe guide to the affinities.
In the Tracheata the archenteron is never, so far as we know, formed by an invagination[215], while in Crustacea the evidence is in favour of such an invagination being the usual, and, without doubt, the primitive, mode of origin.
The mesoblast in the Tracheata is formed in connection with a median thickening of the ventral plate. The unpaired plate of mesoblast so formed becomes divided into two bands, one on each side of the middle line.
In both Spiders and Myriopods, and probably Insects, the two plates of mesoblast are subsequently divided into somites, the lumen of which is continued into the limbs.