Page:Natural History, Mollusca.djvu/57

Rh  a diseased condition of the body, deadly to other creatures. The Leith mussels, he adds, were living in a dock, where we may presume they were nurtured and fattened amid putrescent matters; and Dr. Coldstream, than whom no one is better qualified to decide the point, gave it as his opinion that the liver was larger, darker, and more brittle than in the wholesome fish, and satisfied Dr, Christison that there was a difference of the kind. It must be confessed, however, that these observations leave the question pretty nearly where it was before.

Some peculiar secretions of the Mollusca remain to be noticed. And first the black liquor or ink of the Cuttles and Squids, which has already been mentioned as useful to man, but which is doubtless much more useful to the animals themselves. These animals, when in danger, are known to pour forth from a funnel-like orifice a liquor of a blackish-brown colour in considerable abundance; this fluid, readily diffusing itself and mingling with the surrounding water, produces such a cloud of obscurity as frequently enables the crafty animal to escape, enveloped in the mist of its own making,—as the deities in Homer are represented as concealing their favourite heroes.

Somewhat analogous to this is a secretion of a rich purple hue produced and poured forth under excitement, by those large and naked mollusks, the Aplysiæ. I have found one of these animals, on being put into a vessel of clean sea-water, change the whole to a brilliant purple in a very few minutes; and on the water being renewed even again and again, produce the same result. This was a West Indian species, but there is one found occasionally upon our own coasts which has the