260 ZOOLOGY. 
appeared. The squids or cuttle-fish are very active, some- 
times leaping out of the water and falling on the decks of 
large vessels. They dart rapidly back- 
eo, gt ward by ejecting the water from their 
tT a —y siphon or funnel. 
y The Cephalopods are divided into 
two orders, according to the number of 
their gills. 
Order 1. Tetrabranchiata. — This 
group, in which the gills are four in 
| number, is represented by the Nautilus, 
the sole living representative of a num- 
ber of fossil forms, such as Orthoceras, 
Gontatites and Ammonites. 
Nautilus pompitius Linn. (Fig. 21%), 
and Nautilus umbilicatulus are the: 
only survivors of about 1500 extinct 
or eat tether akescei® species of the order. 
Order 2. Dibranchiata.— The Di- 
branchiates are so called from possessing but two gills, while. 
the Tetrabranchiates had, as in Nautilus, numerous unarmed 
tentacles ; these are now represented by ten (Decapoda). or 
at 
a Fig. 215. Fig. 216. 
Fig. 215,—Development of an unknown cuttle-fish. v, cilia ; y, yolk; mé, man- 
tle beginning to develop. 
Fig. 216.--The same, much farther advanced. a, a’, a”, arms; m, mouth ; br, dr, 
gills ; 7, funnel; , ear; g, optic ganglion ; mt, mantle, the dotted line ending in a 
chromatophore,—After Grenacher. 
arms, provided with numerous suckers. To. 
eight (Octopo 
belong Sptrala, a diminutive cutile, with 
the ten-armed fort 
