Fig. 369. Carapaces of various crabs. 1, Geryon; 2, Corystes; 3, Scyramathia; 4, Paralomis; 5, Lupa; 6, Chorinus.
If we choose, to begin with, such a crab as Geryon (Fig. [369], 1), and inscribe it in our equidistant rectangular co-ordinates, we shall see that we pass easily to forms more elongated in a transverse {745} direction, such as Matuta or Lupa (5), and conversely, by transverse compression, to such a form as Corystes (2). In certain other cases the carapace conforms to a triangular diagram, more or less curvilinear, as in Fig. [4], which represents the genus Paralomis. Here we can easily see that the posterior border is transversely elongated as compared with that of Geryon, while at the same time the anterior part is longitudinally extended as compared with the posterior. A system of slightly curved and converging ordinates, with orthogonal and logarithmically interspaced abscissal lines, as shown in the figure, appears to satisfy the conditions.
In an interesting series of cases, such as the genus Chorinus, or Scyramathia, and in the spider-crabs generally, we appear to have just the converse of this. While the carapace of these crabs presents a somewhat triangular form, which seems at first sight more or less similar to those just described, we soon see that the actual posterior border is now narrow instead of broad, the broadest part of the carapace corresponding precisely, not to that which is broadest in Paralomis, but to that which was broadest in Geryon; while the most striking difference from the latter lies in an antero-posterior lengthening of the forepart of the carapace, culminating in a great elongation of the frontal region, with its two spines or “horns.” The curved ordinates here converge posteriorly and diverge widely in front (Figs. [3] and 6), while the decremental interspacing of the abscissae is very marked indeed.
We put our method to a severer test when we attempt to sketch an entire and complicated animal than when we simply compare corresponding parts such as the carapaces of various Malacostraca, or related bones as in the case of the tapir’s toes. Nevertheless, up to a certain point, the method stands the test very well. In other words, one particular mode and direction of variation is often (or even usually) so prominent and so paramount throughout the entire organism, that one comprehensive system of co-ordinates suffices to give a fair picture of the actual phenomenon. To take another illustration from the Crustacea, I have drawn roughly in Fig. [370], 1 a little amphipod of the family Phoxocephalidae (Harpinia sp.). Deforming the co-ordinates of the figure into the {746} curved orthogonal system in Fig. [2], we at once obtain a very fair representation of an allied genus, belonging to a different family of amphipods, namely Stegocephalus. As we proceed further from our type our co-ordinates will require greater deformation, and the resultant figure will usually be somewhat less accurate. In Fig. [3] I show a network, to which, if we transfer our diagram of Harpinia or of
Fig 370. 1. Harpinia plumosa Kr. 2. Stegocephalus inflatus Kr. 3. Hyperia galba.
Stegocephalus, we shall obtain a tolerable representation of the aberrant genus Hyperia, with its narrow abdomen, its reduced pleural lappets, its great eyes, and its inflated head.
The hydroid zoophytes constitute a “polymorphic” group, within which a vast number of species have already been distinguished; and the labours of the systematic naturalist are constantly adding to the number. The specific distinctions are for the most part based, not upon characters directly presented {747} by the living animal, but upon the form, size and arrangement of the little cups, or “calycles,” secreted and inhabited by the little individual polypes which compose the compound organism. The variations, which are apparently infinite, of these conformations are easily seen to be a question of relative magnitudes, and are capable of complete expression, sometimes by very simple, sometimes by somewhat more complex, co-ordinate networks.
