Tremoctopus carina (male), showing the Hectocotylus (a) in its ordinary position.

Fig. 301. Tremoctopus carina (male), showing the Hectocotylus (a) in its ordinary position.

A male Tremoctopus, seen from the ventral aspect.

Fig. 302. A male Tremoctopus, seen from the ventral aspect. The visceral sac has been laid open and the left half of the mantle turned aside, to show the branchia and the opening of the generative apparatus during the expulsion of the spermatophore. a, sacculus containing the Hectocotylus; b, branchia of the left side, with its branchial heart, c; d, the "bottle," from which the spermatophore (e) is in progress of expulsion; f f, the mantle.

Embryo of Cuttle fish.

Fig. 303. Embryo of Cuttle-fish.

(1611). Among the many interesting phenomena presented by the Cephalopoda, few are more remarkable than the extraordinary power which these animals possess of continually changing their colour in conformity with the varying tints of surrounding objects, affording a means of defence almost as efficient for concealment as the ejaculation of their inky fluid. It is indeed an extremely beautiful sight to witness the nickering hues of one of these animals, that seem to succeed each other with astonishing rapidity. In order to explain the cause of this very curious faculty, it is only necessary to examine with the microscope a portion of integument removed from the living animal, when it becomes at once apparent that the coloured layer of the skin is studded with innumerable pigment-cells (chromatophores), filled with colours that exactly correspond with the hues of the creature's body, and which individually possess a remarkable power of changing their shape and size, and thus modifying, by their contraction and expansion, the coloration of the integument*.

(1612). Interesting as the subject is, our space will permit us to advert but very briefly to the important light which our comparatively recently acquired knowledge of the anatomy of the Cephalopod Mollusca has thrown upon the history of innumerable races of similarly constructed beings long extinct, the remains of which are extensively distributed through a variety of geological strata. The fossil Ammonites for example, the nature of which was previously inexplicable, were at once rendered intelligible by the discovery and description of the Nautilus Pompilius, to which in their structure they were evidently nearly related. The countless petrified remains known by the names of Hamites, Lituites, Orthoceratites, Cirthoceratites, and other allied forms are still representatives of the existing Spirula (fig. 304), which, although its structure even at the present day is but imperfectly understood, is manifestly a Cuttle-fish provided with a camerated shell constructed much after the same plan as that of the Nautilus; and even the Belemnites, so long a puzzle and a mystery to geologists, when restored by the labours of Professor Owen, as represented in fig. 305, unmistakeably confesses its relationship to the Cuttle-fishes we have been describing.

* This wonderful faculty of changing their colours, dependent on a similar organization of the rete mucosum, is possessed by fishes, frogs, and many other animals, as may be demonstrated by simply placing a few small trouts, gudgeons, or minnows in differently coloured earthen pans filled with clear water; and the phenomenon is rendered still more conspicuous by suddenly transferring them from the lighter to the darker coloured vessels, and vice versa.

1. Animal of Spirula, exhibiting the shell in situ. 2. Transverse section.

Fig. 304. 1. Animal of Spirula, exhibiting the shell in situ. 2. Transverse section, and, 3, lateral view of the shell removed from the body.

Belemnite restored: a, cephalic tentacles; b, siphon; e, ink bag; d, e, section of shell.

Fig. 305. Belemnite restored: a, cephalic tentacles; b, siphon; e, ink-bag; d, e, section of shell. The outline exhibits the fin-like expansions of the integument.

(1613). Leaving the Cephalopod Mollusca, we must bid adieu to the fourth grand division of the animal kingdom, and proceed in the next chapter to introduce the reader to beings organized according to a different type, embracing the most highly gifted and intelligent occupants of the planet to which we belong.