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ARTICULATA. 95 
bill, the mouth is surrounded with numerous tentacles retractile into separate 
sheaths, and disposed in eight groups corresponding to the arms of Octopus. 
They are surrounded by a kind of hood, which serves to protect the animal 
when withdrawn into the shell. The two eyes are large and prominent, 
the heart single, with a ventricle and auricle, and inclosed in a large 
pericardium which is connected with the siphuncle. The siphon or funnel, 
which is ventral, corresponds to the part of the shell furthest from the 
centre, that is, the external lip. The fossils named rhyncholites, are the 
beaks of various species of Vautilus and Ammonites. Several other genera, 
with the shell both curved and straight, belong to this family, and among 
the latter is the fossil genus fs aig as, of Beare (not of Lamarck), of the 
older formations. 
Fam. 2. Ammonitide (pl. 76, figs. 14,15). In this family the shell is 
spiral in the same plane, curved, or straight, with the siphuncle at the 
dorsal margin. The partitions which divide its chambers are irregular, with 
the margins digitated or foliated, often in a singular and beautiful manner. 
All the species are extinct, and they are found from the older fossiliferous 
formations to the cretaceous strata. This and the preceding family are 
represented at the present day by the two living species of Vautilus, and 
four of Spirula, a mere remnant of the profusion of species of chambered 
shells which lived in the ancient seas. Upwards of 300 species of the 
genus Ammonites have been described or indicated; and although it is 
probable that many of these belong to varieties and young individuals, with 
every allowance, the genus is very extensive. 
Dryviston III. Articunata. 
The Articulata are named from having the various parts of the body and 
limbs articulated to each other. The nervous system is composed of 
ganglions united by a’double cord, and there is usually a kind of exterior 
skeleton composed of a series of rings protecting the interior parts, and 
serving as points of attachment for muscles. In some cases respiration is 
effected by means of branchis, and in others by trachez or air tubes. 
When limbs are present, there are never fewer than six. 
Authors upon the Articulata are not agreed upon the number and extent 
of the classes into which they should be distributed, but they will be treated 
of in detail, nearly as they are given in pages 11, 12, according to which 
the classes are five, namely 1, Annelida; 2, Cia: 3, Crustacea ; 4, 
Arachnida ; 5, Insecta. The three last of these formed the ae of 
Linneus, a sett which some modern writers wish to maintain according to 
its original extent; and when this is done, the restricted class of insects 
must have a distinct name; as Ptilota (winged insects), applied by 
Aristotle, and Condylopa, applied by Latreille. The Myriapoda will form 
the fifth, and the Insecta the sixth class, in the succeeding pages. 
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