CEPHALOPODA. 457 



posed as to prevent all reflux towards this central heart ; and thus 

 the circuit of the blood, accomplished in this complicated system of 

 blood-vessels, is completed. In Nautilus the lateral sinuses (w, n) 

 are wanting, and the systemic ventricle is of a square shape ; but 

 in other respects the course of the circulation is the same as is 

 above described. 



(499.) In the nervous system of the CEPHALOPODA we may 

 naturally expect to find not only a superiority in the developement 

 of the nervous centres, as compared with the condition of these 

 important masses in the lower Mollusca, but some indications at 

 least of an approximation to that arrangement so eminently cha- 

 racteristic of the vertebrate division of the animal world, to the 

 confines of which we are now gradually approaching ; more espe- 

 cially as in the activity of the movements of these creatures, and in 

 the increased perfection of their senses, we have abundant evidence 

 of the elevated position assigned to them, when contrasted with 

 other mollusks of less carnivorous and rapacious habits. 



The nervous ganglia from whence the muscles and viscera derive 

 their supply are still numerous and widely scattered ; but their size 

 is considerable, and proportioned to the importance of the organs 

 over which they preside. It is to the encephalic portions of the 

 nervous system, however, that we must principally turn our atten- 

 tion if we would rightly estimate this part of their economy ; and 

 these, we at once perceive, have in the class before us attained to 

 such magnitude and importance that they no longer dubiously 

 emulate the brain of a fish, with which it is not difficult to com- 

 pare them. 



In a Cephalopod, the encephalon for so we now may truly call 

 it is enclosed, as has been already noticed, in a distinct cartila- 

 ginous skull, which embraces it on all sides, and defends it from 

 injury. The capacity of the cranial cavity is however more than 

 sufficient to contain the brain ; and, as is the case in fishes, the 

 interspace is filled up with a semigelatinous substance. The 

 brain, however, still forms a ring through which the oesophagus 

 passes ; so that we might with propriety preserve the terms supra- 

 oesophageal and infra-oesophageal ganglia, were these parts not now 

 become so intimately united to each other that they seem fused 

 into a single mass (Jig. 215, a, &), from different portions of which, 

 nerves, serving very different offices, take their origin. 



(500.) In Nautilus the nervous system has been most minutely 

 and critically examined ; and the important deductions to which the 



