V. CEPHALOPODA 



343 



upper and lower buccal ganglia. The large optic ganglia, in the optic 

 nerve arising from the cerebrum and enclosed ventrally in the 'white- 

 body,' a lymphoid mass, are especially characteristic, as are the gang- 

 lia stellata, right and left at the anterior edge of the mande (fig. 356, si), 

 which owe their name to the radiation of fibres to innervate the 

 mantle. An unpaired sympathetic ganglion hes at the junction of 

 stomach and intestine. Cerebral, pedal, visceral and optic ganglia are 

 enclosed in the cephalic cartilage, which has the shape of a ring with 

 wing-like processes. The complicated statocysts lie in the ventral arch 

 of the ring. The sense of smell is highly developed. Apparently it 

 resides in a pair of spots of skin between the eyes and the mantle which 

 are richly supplied with nerves. In the Decapoda these are sunk in pits, 

 in the Octopoda they form papilla;. In Nautilus, which has also two 

 pairs of osphradia, there is a papilla with a ciliated 

 groove, beneath each eye, corresponding to the 

 olfactory organ of the other groups. 



Most noticeable of the circulatory structures is 

 the presence of two kinds of hearts (fig. 356). 

 The systemic heart consists of two (four in Nati- 

 tiliis) auricles receiving arterial blood from the 

 gills, and a median ventricle from which arise 

 anterior and posterior aortfe. Then there is a 

 branchial heart at the base of each ctenidium 

 which receives the blood from the vena cava and 

 pumps it into the gill. Of venje cava; there are 

 an anterior unpaired and two posterior paired 

 trunks, the former dividing and sending a branch 

 to each branchial heart. These trunks are con- 

 nected with the nephridia. The nephridial open- 

 ings (p. 339) lead to two spacious sacs through 

 which the veins pass obliquely. This part of the 

 blood vessels bears venous diverticula which pro- 

 ject into the nephridial sac and are covered with an epithelium of 

 excretory cells. Near its mouth each nephridial sac communicates by 

 a nephrostome with the (usually large) coelom. 



Fig. 357. — Nervous 

 system oiScpia officinalis 

 from the side, ghi, in- 

 ferior buccal ganglion ; 

 gbs, superior buccal 

 ganglion; gc, cerebral 

 ganglion; gp, pedal gan- 

 glion; gv, visceral gan- 

 glion; mb, buccal mass; 

 oe, oesophagus; op, optic 

 gangUon. 



In the Octopoda the ccelom is reduced to the gonads and narrow canals 

 leading from the nephrostome to the gonads and branchial hearts, but else- 

 where there is a well-developed system of connected cavities, consisting of the 

 pericardium around the systemic and branchial hearts and the thin-walled 

 genital sac, one wall of which bears the genital ducts, while on the other the 

 sexual cells arise or the ducts of a separate sexual gland open (fig. 35S). 



