20 A SYNOPSIS OF ANIMAL CLASSIFICATION. 



or tentacles; foot rolled into a 

 sort of funnel. 



Order I. Dibranchiata Gills two; eight or ten arms 



around the mouth, covered with 

 cup-shaped sucking discs; ink-bag 

 present'; shell internal (in Argo- 

 nauta not entirely covered). 



X Loligo, Octopus. 



Order 2. Tetrabranchiata . . . .Gills four, mouth surrounded by 



numerous unarmed tentacles ; ink- 

 bag wanting; a heavy external 

 shell divided into chambers. A 

 single living genus. x Nautilus. 



The present Cephalopods are the few degenerate descendants of a 

 very large and abundant group, which filled the seas in Palaeozoic and 

 Mesozoic times. They possessed, originally, well-developed shells, 

 divided into chambers ; some shells being straight, others spirally coiled. 

 The Tetrabranch, Nautilus, is the only living representative of the great 

 Order Nautiloidea, and still retains a heavy chambered shell. The 

 other great Tetrabranch Order, the Ammonoidea, is entirely extinct. 

 These possessed spirally wound shells with chambers and septa, the latter 

 often fluted and otherwise ornate. The Belemnoidea were Dibranchs. 

 They possessed usually straight shells, and were the ancestors of the 

 modern squid. 



TYPEV. ARTICULATA. 



Segmented bilateral animals, with a chitinous exo-skeleton which 

 varies in thickness from a thin skin to a hard shell, reinforced by 

 mineral salts. Body cavity present and spacious; alimentary canal 

 complete, approximately coinciding with the longitudinal axis; nerv- 

 ous system a double chain of ganglia, typically a pair for each 

 somite; vascular system consists of a longitudinal dorsal vessel, and 

 generally one or more ventral ones, connecting with the dorsal one 

 by commissures. Sexes almost always separate. 



Sub-type I. ANNELIDES Primitive Articulates, worm -like 



in appearance and without jointed 

 limbs or definite somite-complexes. 

 Body cavity divided by dissepi- 

 ments into somitic divisions. 



