MOLLUSCA. I8t 
fig. 99); in the bivalves (Lamediibranchiata), it is mostly fur- 
nished with four membranous processes or “ palpi” (fig. 100, 4) ; 
in others it is furnished with a complicated toothed organ or 
‘“-odontophore” (Gasteropoda, fig. 103, and Pterofoda); and 
lastly, the Cefhalopoda, in addition to an odontophore, possess 
horny mandibles, forming a kind of beak, very like that of a 
parrot. 
The blood is colourless, or nearly so. In the lowest class of 
the Mollusca (Polyzoa) the circulation is carried on by means 
of cilia, and there is no distinct heart nor any definite course of 
the circulating fluid. In the sea-squirts (7zzcata) there is a 
distinct heart, but the structure of this is very simple, consisting 
of a mere tube, open at both ends, so that the course of the cir- 
culation is periodically reversed. In thé higher A/o//usca there 
is a distinct heart, consisting of two chambers, of which one 
(the auricle) receives the aerated blood from the gills, whilst 
the other (the vextricle) drives it through the body. 
Respiration is very variously effected amongst the AZollusca, 
In the Polyzoa (fig. 96) respiration is discharged mainly by the 
crown of ciliated tentacles surrounding the mouth. © In the sea- 
squirts (fig. 97) respiration is effected by a greatly-developed 
pharynx, which is perforated by numerous ciliated apertures. 
In the lamp-shells and their allies (Brachiopoda, fig. 99) the 
long ciliated arms which spring from the sides of the mouth 
seem to be the main agents in respiration. In the Bivalve shell- 
fish, the cuttle-fishes, and most of the Univalves, the breathing- 
organs are in the form of gills or branchiz, adapted for breath- 
ing air dissolved in water. In the remainder of the Univalves 
(eg., snails and slugs) the breathing-organs are adapted for 
breathing air directly, and have the form of an air-chamber or 
pulmonary sac, produced by the folding of a portion of the 
mantle. The air is admitted to the chamber by a round open- 
ing situated on the side of the neck, and capable of being closed 
at will. The lining membrane of the chamber is richly sup- 
plied with blood-vessels, and thus the necessary purification of 
the blood is carried out, 
In accordance with the scattered or rudimentary condition of 
the nervous system, the (/o//usca are not characterised by acute- 
ness of senses, nor by any great power of locomotion. Organs 
of sight exist in some of the lower and many of the higher 
Mollusca, attaining in the cuttle-fishes' (fig. 109) an extremely 
