That mode of it which consists in the union of similar parts originally separate,has been described by Milne-Edwards and others, as exhibited in various Invertebrata;though it does not seem to have been included by them as an essential traitof organic development. We shall, however, see that local integration isan all-important part of this process, when we find it not only in the successivestages passed through by every embryo, but also in ascending from the lowercreatures to the higher. As manifested in either way, it goes on both longitudinallyand transversely; under which different forms we may conveniently considerit. Of longitudinal integration, the sub-kingdom Annulosa * <* I adhereto this name though of late years the two divisions Annelida and Arthropodahave usurped its place. Their kinship as lower and higher is admitted, andthe name is descriptive of both; for the being formed of rings is their mostconspicuous structural trait.> supplies abundant examples. Its lower members,such as worms and next to them myriapods, are mostly characterized by thegreat numbers of their segments; reaching in some cases to several hundreds.
But in the higher divisions -- crustaceans, insects, and arachnids -- thisnumber is reduced to twenty-two, thirteen, or even fewer; while, accompanyingthe reduction, there is a shortening or integration of the whole body, reachingits extreme in the crab and the spider. The significance of these contrasts,as bearing on the doctrine of Evolution, will be clear when it is observedthat they are parallel to those which arise during the development of individualannulose animals. The head and thorax of a lobster form one compact box,made by the union of a number of segments which in the embryo were separable.
Similarly, the butterfly shows us segments so much more closely united thanthey were in the caterpillar, as to be, some of them, no longer distinguishablefrom one another. The Vertebrata again, throughout their successively higherclasses, furnish like instances of longitudinal union. In most fishes, andin limbless reptiles, none of the vertebra coalesce. In most mammals andin birds, a variable number of vertebra become fused to form the sacrum;and in the higher apes and in man, the caudal vertebra also lose their separateindividualities in a single oscoccygis. That which we may distinguish astransverse integration, is well illustrated among the Annulosa in the developmentof the nervous system. Leaving out those most degraded forms which do notpresent distinct ganglia, we find that the lower annulose animals, in commonwith the larva of the higher, are severally characterized by a double chainof ganglia running from end to end of the body; while in the more perfectly-formedannulose animals, the two chains unite into a single chain. Mr. Newport hasdescribed the course of this concentration in insects, and by Rathke it hasbeen traced in crustaceans. During the early stages of the common cray-fish,there is a pair of ganglia to each ring. Of the fourteen pairs belongingto the head and thorax, the three pairs in advance of the mouth consolidateto form the cephalic ganglion or brain. Meanwhile, of the remainder, thefirst six pairs severally unite in the median line, while the rest remainmore or less separate. Of these six double ganglia thus formed, the anteriorfour coalesce into one mass; the remaining two coalesce into another mass,and then these two masses coalesce. Here longitudinal and transverse integrationgo on simultaneously and in the highest crustaceans they are both carriedstill further. The Vertebrata exhibit transverse integration in the developmentof the generative system. The lowest mammals -- the Monotremata -- in commonwith birds, to which they are in many respects allied, have oviducts whichtowards their lower extremities are dilated into cavities, each imperfectlyperforming the function of a uterus. "In the Marsupialia there is acloser approximation of the two lateral sets of organs on the median line: for the oviducts converge towards one another and meet (without coalescing)on the median line; so that their uterine dilatations are in contact witheach other, forming a true 'double uterus.'... As we ascend the series of'placental' mammals, we find the lateral coalescence becoming more and complete....
In many of the Rodentia the uterus still remains completely divided intotwo lateral halves; whilst in others these coalesce at their lower portions,forming a rudiment of the true 'body' of the uterus in the human subject.
This part increases at the expense of the lateral 'cornua' in the higherherbivora and carnivora; but even in the lower quadrumana the uterus is somewhatcleft at its summit."* <* Carpenter's Prin. of Comp. Phys., p. 617.>
Under the head of organic integrations, there remain to be noted anotherclass of illustrations. Whether the Annulosa referred to above are or arenot originally compound animals, it is unquestionable that there are compoundanimals among other classes of invertebrates: integration is displayed notwithin the limits of an individual only but by the union of many individuals.
The Salpidae are composite creatures having the shape of chains joined moreor less permanently; and Pyrosoma shows us a large number united into a cylinder.
Moreover in the Botryllidae the merging of the individualities goes so farthat instead of having separate skins they are enclosed within a common skin.
