84 COMPARATLl'E ANATOMY AND PHYSIOLOGY. 



Mollusca form three very distinct branches or phyla, 

 the common ancestor of which is to be sought for 



o 



only in a simple worm. Of equal valne with 

 these is another phylum, which may be most conve- 

 niently spoken of as that of the Chordata, distin- 

 guished from the rest by the association of two 

 characters, the temporary or permanent possession 



of a rod underlying the central 

 dorsally-placed nervous system, 

 and the similarly temporary or 

 permanent possession of clefts 

 or passages at the sides of the 

 head and neck, which open to 

 the exterior (visceral clefts). 

 Either one of these cha- 

 racters may be seen in certain 

 members of that heterogeneous 

 mob, which, parti}'" from the 

 nature of things, and partly 

 from the imperfect condition of 

 our knowledge respecting them, 

 must be retained in the group 

 of Vermes. 



Among, or standing near to, 

 the Platyhelminthes, are some 

 elongated, free-swimming, ma- 

 rine forms which are known as the IVemertiiiea. 

 These worms are provided with a dorsal proboscis, which 

 is enclosed in a sheath. The relations of this proboscis 

 to its sheath are shown in Fig. 42 A, while Fig. 42 B ex- 

 hibits in diagrarnatic form the relation of certain parts 

 in one of the lowest of fishes (the lamprey) ; a comparison 

 of the relations of these structures (proboscis and its 

 sheath on the one hand, and chorda dorsalis on the 

 other) with (a) the dorsal surface of the body and (#) 

 the digestive tract, reveals very striking resemblances, 

 which come to be of still greater significance when we 



F g. 41. The Common 

 Cuttlefish. 



