EVOLUTION OF THE ATYPICAL FISH 201 



" The funnel (Fig. 66,/), is placed on the lower surface of the body, 

 with its anterior extremity projecting beyond the mantle, while it 

 opens behind into the pallial chamber. It serves for the elimination 

 of the water which has been used in respiration, and the outgoing 

 currents also carry with them the excretions of the kidneys and of 

 the ink-sac, together with the fseces. By the contractions of the 

 mantle, the water contained in the pallial sac can also be driven through 

 the funnel in a succession of jets, driving the animal backwards through 

 the water. . . . The circulatory organs consist of a central ventricle, 

 into which the aerated blood from the gills is poured by two laterally 

 placed auricles developed on the branchial veins where they leave 

 the branchise. . . . The respiratory organs are in the form of two 

 (Cuttlefishes) or four (Nautilus) plume-like gills, placed symmetrically 

 on the sides of the body within the pallial sac . . . the necessary 

 respiratory currents are maintained by the alternate contractions 

 and expansions of the muscular walls of the mantle-sac. In each 

 expansion the water finds its way into the pallial chamber by the 

 opening between the rim of the mantle and the neck ; and in each 

 contraction it is expelled through the tube of the funnel, which is 

 so constructed as to allow of the egress but to prevent the ingress 

 of the water. The central nervous system consists of the three normal 

 pairs of ganglia . . . but these are aggregated to form an oesophageal 

 collar (Fig. 66, ng). . . . The great oesophageal nerve-collar is pro- 

 tected by a cartilaginous plate which foreshadows the cranium of 

 the Vertebrata. . . ." (Nicholson.) 



It will be noticed that most Cephalopods are able to 

 move forwards by the action of lateral fin-fringes, but that 

 when alarmed or desiring to move with speed they move 

 in a backward direction by ejecting water strongly from 

 the mantle sac. In the latter case the advancing end of the 

 animal is that which in the typical Fish has become 

 the anterior ; and the fact that backward movement is 

 more efficient than forward in the Cephalopod, and also 

 the method of the movement's production, clearly reflect 

 the serial-medusoid derivation of the organism. The ejection 

 of water through the funnel is due to "wall-contractions" 

 which represent ancestral serial-medusoid contractions, and, 

 as in the case of medusoids, the mechanism of locomotion 

 is here also that of respiration. In the common Octopus 

 backward movement is also obtained by additional means 

 representing ancestral bell-contractions ; for the organism 

 expands and contracts the fringe which bears the tentacles, 

 these being drawn behind the head to be thrown forwards 

 immediately afterwards. 



