ARTICüLATA. 95 



bill, the month 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, M^hich serves to protect the animal 

 when withdrawn into the shell. Tlie 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 lij). The fossils named rhyncliolitea^ are the 

 beaks of various species of Nautilus and Ammonites. Several other genera, 

 with the shell both curved and straight, belong to this family, and among 

 the latter is the fossil genus Orthoceras^ of Breyn (not of Lamarck), of the 

 older formations. 



Fam. 2. Ammonitidm {pi. IQ^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 Nautilus^ 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. 



Division III. Articulata. 



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 branchias, and in others by tracheae 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 live, namely 1, Annelida ; 2, Cirrhojpoda ; 3, Crustacea ; 4, 

 Arachnida ; 5, Insecta. The three last of these formed the Insecta of 

 Linnaeus, a term 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. 



299 



