158 SUPPLEMENT 
without lobules, or digitation, or tentacular cirrhi, as in sepia 
and the neighbouring genera; but it often happens that the 
upper edge advances a little to form a sort of shelter for the 
head, as in onchidia, and even in limax, or that it is consider- 
ably prolonged by the addition of a thick muscular appendage, 
in the form of a cornet, open below, but constituting a com- 
plete tube, more or less elongated, and serving as an introduc- 
tion for the water into the branchial cavity. ‘This is obser- 
vable in all the siphonobranches, in which the aperture of the 
shell is emarginated or siphonated. 
We find a small number of species of mollusca of this class, 
in which the lateral edges of the mantle are lobate or digitate; 
but there are a few more which have them furnished with 
fringes or tentacular cirrhi. The cervicobranches, and espe- 
cially the patella and haliotides, are the species which more 
particularly present this character. 
But it is especially in the class of the acephala that the 
marginal cirrhi of the mantle acquire the greatest develop- 
ment, both in size and number. In the lime, for example, 
they are almost small cylindrical tentacula, forming a quadru- 
ple cordon round the edges of the mantle. In pecten, the 
cirrhi, which are also large and numerous, are intermixed with 
small oval plates, iridescent, in the form of eyes, regularly 
intervalled, and the use of which is completely unknown. 
In this same class of animals, the edges of the mantle pretty 
frequently present lobules, or digitations, more or less marked; 
and in the species in which the labial lobes are more or less 
completely united, they are so behind, by means of one or two 
muscular tubes, entirely contractile, distant or not, short, or 
very much elongated, whose orifices are often furnished with 
cirrhi, and assume an almost radiated arrangement. These 
tubes seem, one, or the ventral, for the introduction of food, 
the other, or the dorsal, for the ejection of excrement. In the 
