804 ZOOLOGY sect. 



Nautilus itself. The Ammonites are mainly Mesozoic, the repre- 

 sentatives found in the earlier rocks (from the Upper Silurian 

 onwards) being few in number and simpler in structure than 

 the more typical later forms. The oldest fossil representatives 

 of undoubted Dibranchiata belong to the extinct order of the 

 Belemnites ; which flourished in the Mesozoic period from the 

 Trias to the Cretaceous, and survived in scanty number into the 

 Tertiary. Unlike the Tetrabranchiata, the Dibranchiata would 

 appear to have reached their maximum at the present day. 



The mutual relationship of the various groups of Cephalopoda 

 are indicated, as nearly as the information at our disposal will 

 allow, in the following diagram (Fig. 704). 



Decapoda 



Belemnites 

 Naurlloids \ / ^Ocropoda 



Ammonites 



Fig. 704. — Diagram to illustrate the relationships of the groups of Cephalopoda. 



General Remarks on the Mollusca. 



The Mollusca, like the Arthropoda, form an extremely well- 

 defined phylum, none of the adult members of which approach 

 the lower groups of animals in any marked degree. There are, 

 however, clear indications of affinity with " Worms," especially 

 in the frequent occurrence of a trochophore stage in develop- 

 ment, in the presence of nephridia, and in the occurrence, in 

 Amphineura and some of the lower Gastropoda, of a ladder-like 

 nervous system resembling that of some Turbellaria and of the 

 most worm-like of Arthropods — Peripatus. The head-kidneys or 

 primitive nephridia of the molluscan and annelid trochophore 

 are practically identical, and are probably homologous with the 

 various types of nephridial tubes found in " Worms " from Platy- 

 helminthes to Chsetopoda. 



If the occurrence of the trochophore be taken as a guide towards 

 the ancestry of the Mollusca, it need not necessarily be regarded 

 as leading back to the Annulata. In fact the presence of not 

 more than a single pair of nephridia (and of ctendia) in all with 



