374 . COMPARATIVE ANATOMY. 



off, and this generally passes backwai^ds. This, which is the general 

 mode of arrangement, shows distinctly how the Cephalopoda agree 

 with the two other classes, while it points to their having a close 

 affinity to those MoUusca which are distinguished by the possession 

 of two auricles. 



The cephalic artery fii'st of all gives off large branches to the 

 mantle, and some to the intestinal tract and to the funnel; when it 

 reaches the head it gives off the optic arteries and supplies the oral 

 regions, and divides into larger branches, the number of which 

 depends on that of the arms. In some of the Cephalopoda the 

 brachial arteries spring from a circular vessel, which is developed 

 around the commencement of the cesophagus. The abdominal artery 

 is more varied in character; in some of the Sepiados (Fig 202, a') 

 and Loliginidte, it arises opposite to the cephalic artery, and has, 

 therefore, the same relations as the visceral artery in the Lamelli- 

 branchiata ; but, in the Octopoda, it arises fi'om the anterior region 

 of the heart, close to the aorta (Fig. 199) ; it very soon breaks up 

 into several branches for the enteric tube and generative organs. In 

 the Loliginidfe, however, it gives off two additional branches for the 

 fins ; in Ommastrephes a special enlargement (which is perhaps an 

 .accessory organ of circulation) may be observed on these vessels. 



All over the body the terminal branches of the arteries communi- 

 cate with the veins by means of a well- developed system of capil- 

 laries. In the greater part of the body, at any rate, this system 

 takes the place of the lacunar blood-passages which were found in 

 the other Mollusca, and of which it seems to be a differentiation. 



The venous roots from the capillaries are collected into larger 

 trunks, which have sometimes the characters of true veins, and are 

 sometimes widened out into large spaces, so that they are inter- 

 mediate between true vessels and mere lacuna. Of the more special 

 characters of the venous system we have to note that the brachial 

 veins are united into a circular sinus, placed in the head ; this is 

 supplied also by neighbouring smaller venous trunks, while it 

 gives off a large ha;mal canal (vena cephalica, or great vena cava) 

 (Fig. 202, vc), which passes backwards to the branchial region. At 

 the gills it breaks up into two (Dibranchiata) or four (Tetrabran- 

 chiata) venous trunks ; these take up the other veins which come 

 from the mantle and viscera {vc") and pass to the base of the gills. 

 In the Dibranchiata the branchial arteries acquire a muscular in- 

 vestment and form a contractile portion, or branchial heart 

 (Fig. 202, vc ), which pulsates rapidly, and acts as an accessory organ 

 of the circulation. Special appendages are attached to the branchial 

 artery in front of these branchial hearts ; these, which are diverti- 

 cula of the walls of the vessels, are bathed by the venous blood, 

 which passes into the branchife, in just the same way as are the organs 

 of Bojanus in the Lamellibranchiaita (vide Excretory Organs, § 289). 



Although the venous blood-receptacles of the Cephalopoda which 

 we have described may be regarded as a venous system, provided 

 with closed walls, true blood lacuna are not absent. They are dis- 

 tributed in just the same way as in the other classes of the Mollusca. 



