100 ZOOLOGY. 



that lias come from the various parts of the body through 

 these organs. These cavities are called branchial hearts. 

 They are wanting in the nautili. In the cuttle-fishes, &c., 

 the veins and arteries are connected by capillaries. It is 

 worthy of note, that while the blood in the heart of a 

 fish is venous, the heart of a mollusc contains arterial 

 blood. 



132. Respiration. All cephalopods breathe by gills or 

 branchiae. These are plume-like organs, placed on the 

 sides of the animal in a cavity formed by the mantle 

 which opens in front on the under surface of the body. 

 When the mantle expands, water is forced into this 

 cavity ; when it contracts, it is ejected through the 

 funnel, which is provided with a valve, allowing water to 

 pass towards the head, but not otherwise. The reaction 

 produced by the escape of this water causes the animal 

 to move in the opposite direction, that is, tail foremost. 

 Thus, in the cephalopods, as in many other animals, 

 the functions of locomotion and respiration are intimately 

 associated. 



The cuttle-fish tribe and the paper nautilus are 

 each provided with two gills ; the pearly nautilus with 

 four. 



133. The nervous system consists, as in the other Mol- 

 lusca, of three ganglia united by nervous cords the cere- 

 bral, pedal, and parieto-splanchnic ganglia. The cerebral 

 ganglia are large, and are surrounded by a cartilaginous 

 ring which may be considered a rudimentary skull. 

 This is the nearest approach to an internal skeleton 

 found in any invertebrate animal. There are two large 

 eyes situated on the sides of the head. 



134. Development. The Cephalopoda are unisexual 

 animals. Their ova are of large size comparatively. In 

 the cuttle-fishes one of the arms of the male is adapted for 

 conveying to the gill chamber of the female the sperm-cells 

 which have previously been formed into a mass by a 

 glutinous substance. In the common octopus or poulpe, 

 the third arm on the right side has at first the appearance 



