267 
strongly refracting point near each extremity. These 
particles vary much in size, and are sometimes curiously 
combined, so as to appear double, or assume the form of a 
star or cross/' * 
The Mouth and Appendages. — In the centre of the circle 
of arms which crown the head lies the mouth, which in the 
Decapoda is provided with an investing and surrounding skin 
(buccal membrane), covering the exterior of the mouth, and 
capable of being pouted out in the form of a large funnel ; 
this organ is wanting in the Nautilus, and in the long-armed 
Octopods. In all the mouth is furnished with two lips, one of 
which is external, the other internal, fleshy, and covered with 
papillae. Within the cavity of the mouth are situated the man- 
dibles, which in shape are not unlike those of a parrot, and by the 
aid of strong muscles act in a vertical direction. These 
mandibles, in all living species except the Nautilus, are of a 
horny or chitinous consistency throughout, but in the Nautili 
their extremities are tipped with a dense calcareous substance ; 
the anterior mandible is always the shortest, and is over- 
lapped by the posterior one (the reverse order to what obtains 
with birds). The tongue and Odontophorey or lingual ribbon, 
perform an important part in the economy of the animal. 
The anterior portion, or tongue, is fleshy and sentient, the 
posterior portion resembles the lingual ribbon of the Gastero- 
poda, being armed with transverse series of silicious teeth. 
The Aliment a rf/ Canal. — The mouth cavity opens into a 
long gullet or oesophagus, and two or four salivary glands 
are connected with it in the Dihranchiata ; in the Nautili 
these glands are very feebly developed, if, indeofl, they be 
present at all. The stomach is situated at the lower part 
of the body, and at its anterior end it is connected with 
the oesophagus, whilst its posterior portion is joined to the 
• " On tlio Anatomy of Nautilus umbilicatus," &c., J. D. MacdouaUl, Loudon, 1850. 
19 
