Fig. 371. a, Campanularia macroscyphus, Allm.; b, Gonothyraea hyalina, Hincks; c, Clytia Johnstoni, Alder.
For instance, the varying shapes of the simple wineglass-shaped cups of the Campanularidae are at once sufficiently represented and compared by means of simple Cartesian co-ordinates (Fig. [371]). In the two allied families of Plumulariidae and Aglaopheniidae the calycles are set unilaterally upon a jointed stem, and small cup-like structures (holding rudimentary polypes) are associated with the large calycles in definite number and position. These small calyculi are variable in number, but in the great majority of cases they accompany the large calycle in groups of three—two standing by its upper border, and one, which is especially variable in form and magnitude, lying at its base. The stem is liable to flexure and, in a high degree, to extension or compression; and these variations extend, often on an exaggerated scale, to the related calycles. As a result we find that we can draw various systems of curved or sinuous co-ordinates, which express, all but completely, the configuration of the various {748} hydroids which we inscribe therein (Fig. [372]). The comparative smoothness or denticulation of the margin of the calycle, and the number of its denticles, constitutes an independent variation, and
Fig. 372. a, Cladocarpus crenatus, F.; b, Aglaophenia pluma, L.; c, A. rhynchocarpa, A.; d, A cornuta, K.; e, A. ramulosa, K.
requires separate description; we have already seen (p. [236]) that this denticulation is in all probability due to a particular physical cause.
Among the fishes we discover a great variety of deformations, some of them of a very simple kind, while others are more striking and more unexpected. A comparatively simple case, involving a simple shear,
| Fig. 373. Argyropelecus Olfersi. | Fig. 374. Sternoptyx diaphana. |
is illustrated by Figs. [373] and [374]. Fig. [373] represents, within Cartesian co-ordinates, a certain little oceanic fish known as Argyropelecus Olfersi. Fig. [374] represents precisely the same outline, transferred to a system of oblique co-ordinates whose {749} axes are inclined at an angle of 70°; but this is now (as far as can be seen on the scale of the drawing) a very good figure of an allied fish, assigned to a different genus, under the name of Sternoptyx diaphana. The deformation illustrated by this case of Argyropelecus is precisely analogous to the simplest and commonest kind of deformation to which fossils are subject (as we have seen on p. [553]) as the result of shearing-stresses in the solid rock.