Page:Natural History, Mollusca.djvu/142

 pinnæ, and pinnulæ, are all dilated inwardly, so that the stem, which is narrow and slender in one aspect, is wide in another; and the pinnæ are the triangular laminæ, whose wrinkles are in fact the pinnulæ. The organ is connected with the bottom of the lateral groove, for about two-thirds of its length, by a membrane. The plume can scarcely be recognised in its two aspects, even though examined again and again in quick succession. It appears very sensitive, and changes much in appearance by its various degrees of contraction and expansion.

The mantle contains in the centre of its substance an oval shallow cavity, within which lies, quite free and unattached, a shield-like shell of the same form, so delicate in its texture as to be almost membranous, with a very slight indication of a spire at one extremity. The position of the shell is indicated externally by a dark cloudy spot in the middle of the back; and on an incision being made in this part, the shell falls out.

The warmer seas produce the largest and most beautiful species of this genus, some of which are marked with bright colours. They are found swimming in the open ocean, and crawling on the rocks or weeds of the coast, and specimens have been dredged, from various depths to thirty fathoms, on stony bottoms and beds of sea-weed. We have two native species, neither of which can be considered common. The rarer of these, Pleurobranchus plumula, is found on our south-western coast, where it was first discovered by that eminent zoologist, Colonel Montagu. I had recently the good fortune to find two specimens in a rocky cove near Torquay, both of which lived in captivity for some weeks.