206 THE LAMELLIBRANCHIA 
is only furnished with a pair of labial palps on each side. The foot 
is ventral, generally adapted to burrowing and without a plantar 
surface. The two pallial lobes are united by one or two transverse 
muscles which close the two valves of the shell. There are two 
lateral and symmetrical ctenidial branchiae under the mantle ; their 
distal extremities are directed posteriorly and their filaments may 
exhibit an extreme degree of concrescence, either among themselves 
or with the mantle or with the visceral mass. 
I. GENERAL DESCRIPTION AND EXTERNAL CHARACTERS. 
The mantle consists of two thin tegumentary lobes attached 
to the trunk dorsally, and extending over the sides to the ventral 
surface so far that they can be brought together below the foot. 
Thus the whole of the animal’s body may be covered by the mantle. 
In structure, the mantle is normally rather thin, and there is only 
some connective tissue and a few muscular fibres between the internal 
and external layers of epithelium. In some few cases the gonads 
may extend into the mantle: into both lobes in the Mytilidae, into 
the right lobe only in the Anomiidae. The internal surface may 
present glandular modifications, of which the most important are the 
hypobranchial glands characteristic of 
0 the Protobranchia: they are situated pos- 
teriorly beyond the gills. The margins 
fra. LY A) su” of the mantle normally present redupli- 
oe cations, generally three in number 
_ (Fig. 187, pa’, pa", pa’), and in the 
2 Pectinidae the most internal of the three 
is turned inwards to form the ‘‘ velum ” 
pa (Fig. 235). The margins of the mantle 
of Lamellibranchs are frequently fur- 
nished with glands, pigment spots, and 
various sensory organs in the form of 
tentacles, and even of eyes. The pig- 
mentation of the pallial border is due 
to the combined action of the light and 
Fic. 187. the oxygen of the surrounding medium, 
Kellya suborbicularis, Montagu, 2nd is most conspicuous at those points 
eee see cone itiant Where the respiratory fluid enters the 
orifice; 0”, pedal orifice; o”, ex- pallial cavity. 
Racat aa ie eye aa The edges of the two pallial lobes 
te eset Feel may remain free throughout their 
edges. (After Deshayes.) extent; this is the case in Nucula, 
the Anomiidae, the Arcidae (Fig. 188) 
the Trigoniidae, and the Pectinidae (Fig. 235). In all other 
Lamellibranchia the pallial lobes are partially united by the 
ad \: fo) mt 
