130 TECTIBRANCHIATA.—PLEUROBRANCHIDA. 
pinne, and pinnulee, are all dilated inwardly, so that 
the stem, which is narrow and slender in one aspect, 
is wide in another ; and the pinne are the triangular 
lamine, whose wrinkles are in fact the pinnule. 
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 sub- 
stance an oval shallow cavity, within which hes, 
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, Pleuro- 
branchus 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. 
