260 



ZOOLOGY. 



jiS^-'v 



appeared. The sqnids or ciittle-fisli are rery active, some- 

 times leaping out of the water and falling on the decks of 

 large Tessels. They dart rapidly back- 

 ward by ejecting the water from their 

 siphon or f unneL 



The Cephalopoda 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 Xmitilus, 

 the sole living representative of a num- 

 ber of fossil forms, such as Orihocerax, 

 Goniatites and Ammonites. 



Xautilus pompilius Linn. (Fig. 317), 

 and JSlautilus umbilicatulus are the 

 only survivors of about 1500 extinct 

 species of the order. 



Order 2. DibrancMata. — The Di- 

 branchiates are so called from possessing but two gUls, whUe 

 the Tetrabranchiates had, as in Xautilus, numerous unarmed 

 tentacles ; these are now represented by ten (^Decapodd). or 



Fig. 214.— Same as Pig. 

 213, but farther advanced. 



Fig. 21S. 



riG. 216. 



Fig. 215.— Development of an unknown cnttle-fish. r. cilia ; y, yolk; mU man. 

 tie be^^inning to develop. 



Fig. 216.— The same, mnct farther advanced, a, a', a", arms ; m, month : &r. br. 

 gills : /', funnel ; h, ear ; g, optic ganglion ; rrU, mantle, the dotted line ending in a 

 chroniatophore.— Afrer Grenacher. 



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

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



