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posteriorly, to form a tube (the siphon or funnel), open at 

 both ends, which would serve to conduct the water outwards 

 from the subpallial chamber. The front of the foot, sur- 

 rounding the mouth, must have become well developed, and 

 drawn out on each side into four or five projections. The 

 other systems remained in their primitive typical condition. 



This ancestral line divided into two branches, the one 

 leading to the Tetrabranchiata {Nautilus and extinct forms, 

 e.g., Ammonites), and the other, after a considerable amount 

 of further evolution, to the Dibranchiata, including the 

 ordinary cuttlefishes. The Tetrabranchiata possess two pairs 

 of ctenidia and two pairs of nephridia, and the epipodia are 

 not completely united. The lobes of the front part of the 

 foot bear numerous tentacular processes, but no suckers. A 

 large external shell, divided into a series of chambers, is 

 present. 



The ancestral Dibranchiata after separation from the 

 Tetrabranchiata must have undergone some further changes. 

 The fusion of the epipodia became complete. The lobes of 

 the fore-foot developed rows of suckers (acetabula), and the 

 shell became enclosed in folds of the mantle so as to be 

 internal. The nervous system and sense organs became 

 more highly evolved, and finally, an "ink" sac was developed. 

 The Dibranchiata are divided into two groups (Octopoda and 

 Decapoda) according to the number of processes developed 

 from the foot surrounding the mouth. The very long line 

 occupied by the series of ancestral Cephalopoda indicates 

 the extensive modifications the group has undergone during 

 its evolution. The Dibranchiata occupy the highest point in 

 the Mollusca and are very far above the place of origin of the 

 phylum from the Vermes. This shows the great range of 

 organisation which is found amongst the Mollusca. 



The Echinodermata, it has already been seen, probably 



