In some species the lower portion of the vessel-mark is reduced to small dots, as in _L. cambrica_, _fluviatilis_, _piratica_, and others; and the stages are very clear. Starting with the isolated chamber-marks, as in _L. allodroma_, Fig. 5, we get, firstly, a set of spots, as in _L. picta_, which, in the female, Fig. 16, are still connected with the chamber-marks, but in the male, Fig. 17, are isolated. This leads us, by easy steps, to such forms as _L. latitans_, Fig. 14, which consists of a double row of spots upon dark stripes. The intimate connection thus shown to subsist between the characteristic decoration of the abdomen of spiders, and the shape of the important dorsal organ beneath, seems to be strong evidence of effect that internal structure may have upon external decoration.[37] The cephalothorax of spiders, being covered with a hardened membrane, does not show such evidence clearly, for it appears to be a law that the harder the covering tissue, the less does it reflect, as it were, the internal organs. The hard plates of the armadillo are thus in strong contrast to the softer skins of other animals. Nevertheless, there does appear, occasionally, to be some trace of this kind of decoration in the cephalothorax of certain spiders, though it would be hard to prove. The blood vessels of this part (see Fig. 9), though large, are not nearly so prominent as the great dorsal vessel. The chief artery enters the cephalothorax as a straight tube, forks, and sends branches to the limbs, palpi, and eyes. In many species, notably in the genus _Thomisus_, a furcate mark seems to shadow the forked aorta. This is best shown in _T. luctuosus_, Plate VIII., Fig. 11. Moreover, in this and other genera, lines frequently run to the outer pair of eyes, which alone are supplied with large arteries, see Fig. 9. However this may be, it is certain that the entire decoration of spiders follows structural lines, and that the great dorsal vessel has been emphasized by the peculiar pattern of the abdomen. [Illustration: Plate VIII. SPIDERS.] [35] Elements of Comparative Anatomy, by C. Gegenbaur. Translated by Jeffrey Bell and Ray Lankester, 1878, p. 285. [36] Spiders of Great Britain and Ireland, J. Blackwell. Ray. Soc., 1861. [37] The decoration of many of the Hoverer flies and wasps is of a similar character. CHAPTER XIII. COLOURATION OF INVERTEBRATA (_Continued_).