The Classification of Animals. 193 



present, which may be external and internal. The foot has 

 grown round the head ; it is broken up into a large number of 

 lobes, the tentacles. In the mantle-cavity is a muscular pro- 

 jection known as the siphon, and probably the equivalent of the 

 epipodia of other Molluscs. The heart is well developed with 

 two or four auricles ; there are one or two pairs of nephridia 

 and of gills. The Cephalopoda are divided into the Tetra- 

 branchiata and the Dibranchiata. 



Tetrabranchiata : these are the Nautilus and the extinct 

 Ammonites. They have a large external shell, coiled or 

 straight, two pairs of nephridia, of auricles, and of gills. The 

 siphon is incomplete, its edges not meeting. 



The Dibranchiata includes all the squids, Octopis, Argo- 

 nauta, Spirula, etc. The shell is generally internal, but in the 

 paper Nautilus, Argonauia, external. The arms bear suckers 

 which are absent in the Tetrabranchiata. The gills, nephridia, 

 and auricles are but a single pair. The siphon forms a com- 

 plete funnel. An ink-sac is present. 



VIII. Arthropoda. 



The Arthropoda are animals in which the body is clearly 

 segmented, and provided with segmented appendages. They 

 are bilaterally symmetrical, with a hard exoskeleton. The 

 nervous system is on the Annelid plan, a brain connected by a 

 circum-oesophageal commissure with a ventral ganglionated 

 chain. The coelom is so much reduced that in many forms it 

 seems to be altogether absent. There are never cilia, except- 

 ing only in Peripatus. The Arthropoda are divided into two 

 primary divisions : the Crustacea, breathing by means of gills, 

 and the Tracheata, which breathes by means of tracheae, or 

 invaginated " lungs." 



I. CRUSTACEA. 



The Crustacea are typically aquatic Arthropods, breathing 

 by means of gills or by the general surface of the body. There 







