216 



INVEETEBEATE ANIMALS. 



known, except that it must certainly serve to maintain the vitality 

 of the shell. 



Of the fossil Tetrahranchiata the most important are the Ortho- 

 cerata and the Ammonites. The Orthocerata played a very important 

 part in the seas of the Palseozoic or Ancient-life period of the earth's 

 history, in which they apparently filled the place now taken by the 

 predaceous Cuttle-fishes. They agreed with the Nautilus in having 

 a many-chambered shell, divided by curved partitions, perforated 

 by a tube or siphuncle. The shell, however, differed from that of 

 the Nautilus in not being curved or coiled up, but in being straight. 

 In other nearly allied forma the shell was bent or even partially polled 

 up, bvit never so completely as in the true Nautili. Many of the 



Pig. 155.— The Pearly Nautilus (Nautilus pompilius). a Mantle; h Its dorsal fold; 

 c Hood ; Eye ; t Tentacles ; / Funnel. 



Orthocerata were of small size, but some of them were colossal, shells 

 having been found of six or seven feet in length, and as thick as the 

 body of a man. 



The Ammonites (fig. 156), with a number of allied forms of varied 

 shapes and beautiful structure, appear to have taken the place of the 

 Naittilidce, to a great extent, in the seas of the Secondary period ; 

 at which time, too, Dibranchiate Cephalopods first made their ap- 

 pearance. The true Ammonites resembled the Nautilus in having 

 a many-chambered shell, which was coiled up into a spiral, but the 

 position of the siphuncle was different, and the partitions or septa 

 between the various chambers of the shell were wonderfully folded 



