280 



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- 

 ward by ejecting the water from their 

 siphon or funnel. 



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, 

 Goniatites and Ammonites. 



Nautilus pompilius Linn. (Pig. 222), 

 and Nautilus umbilicatulus are the 

 only survivors of about 1500 extinct 

 species of the order. 



Order 2. Dibranchiata. The Di- 

 Iranchiates 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 



-of 



Fig. 219. Same as Fig. 

 218, but farther advanced. 



FIG. 221. 



Fig. 220. Development of an unknown cuttle-fish, v, velum ; y, yolk ; ml, man- 

 tle beginning to develop. 



Fig. 221. The same, much farther advanced, a, a', a", arms ; m, mouth ; br, 6r, 

 gills ; /, funnel ; A, ear ; g, optic ganglion ; mt, mantle, the dotted line ending in a 

 ifcromatophore. After Grenacher. 



eight (Octopoda) arms, provided with numerous suckers. To 

 the ten-armed forms belong Rpirula, a diminutive cuttle, with 



