84 Systematic View. 



on the dorsal side. Nervous system as in Annelida. Highly 

 specialized optic organs (compound eyes). Segmental organs always 

 much reduced in number or entirely wanting. 



Phylum 7. Mollusca .—Bilaterally symmetrical unsegmented 

 animals. A ventral muscular foot. A fold of skin^ the mantle, 

 covering part of the body. No continuous cuticle. A part of the 

 integument secretes a shell which is closely adherent to the animal 

 at certain points. Body-cavity. Dorsal heart. A pair of ganglia 

 above and beneath the pharynx, no ventral nerve cord. Usually a 

 lingual ribbon armed with rows of chitinous teeth. Segmental 

 organs reduced in number. 



Phylum 8. Vertebrata . — Bilaterally symmetrical animals ; 

 certain parts of thejbody (skeleton, musculature) segmentally arranged. 

 Usually two pairs of limbs, never more. Body-cavity. Ventral heart. 

 Central nervous system in the form of a continuous thick-walled 

 tube along the dorsal side, generally enlarged anteriorly. Beneath 

 this an elongate rod, the chorda dorsalis (notochord), which forms 

 the basis of the usually highly specialised endoskeleton. — Appendix : 

 Tunicata. 



