xc Introduction. 



branchial hearts, developed upon the great afferent branchial veins. 

 In the Bihrancliiata the peripheral circulatory system appears to be 

 closed, consisting" of arteries connected by capillaries, in many 

 organs at least, with the veins or great venous sinuses into which 

 the veins expand. In the TetrahrancMata the blood appears to be 

 more widely distributed throughout the various perivisceral cham- 

 bers than it is, according to Mr, Hancock, in the D'lbrancldata ; 

 and as the external water finds in the former free access to the 

 various perivisceral cavities, it would appear that it may thus 

 come, as it does, according to some authorities, in the latter order 

 also, to mix directly with the blood. The gills are in the Dibran- 

 chiate order attached in their entire length to the interior of the 

 mantle cavity, upon the contraction of the muscular walls of which 

 they are, in the absence of any cilia upon their external surface, 

 dependent for fresh supplies of water and aeration. The gills of the 

 Nautili differ from those of the Bihrancliiata not only in their 

 number, but also in being attached only at their bases. The renal 

 organs take the shape of bilaterally symmetrical spongy appendages 

 to the stems into which the vena cava divides, and which carry its 

 blood to the similarly bilateral gills. Certain orifices in the bran- 

 chial chamber exist, by means of which the secretion of these 

 organs can more or less directly find its way into the external 

 water. 



The three tj-^^ical pairs of excitomotor ganglia are readily recog- 

 nizable in the Cephalopoda as in all other Mollusca proper, the 

 anterior position of the pedal ganglia and of their cerebro-pedal 

 commissure to the visceral or parieto-splanchnic ganglia, and their 

 cerebro-visceral commissure, being as readily demonstrable in these, 

 the highest, as in the Lamellibranchiata, the lowest of the Mollusca 

 proper. The accessory nerve-systems, however, which this latter 

 class does not possess, attain a high development in the paired 

 stomatogastric, parietal, and branchial ganglia, as also in the un- 

 paired stomachal and other visceral ganglia of Cephalopoda. An 

 olfactory organ appears to exist in Cephalopoda in addition to the 

 highly-developed eyes and the auditory organs. 



The Cephalopoda are always dioecious. The reproductive glands 

 differ from most of their other oi'gans in not being bilaterally 

 symmetrical, and they differ from those of all other Invertebrata, 

 except certain of the Vermes, in setting free their respective 



