396 MOLLUSC A. 



7. Three pairs of ganglia with which three pairs of sense- 

 organs are connected almost always occur: a, cerebral ganglia 

 and eyes; b, pedal ganglia and otocysts; c, visceral ganglia and 

 osphradia (olfactory). 



8. The heart is dorsal and arterial; it is enclosed in a peri- 

 cardium (reduced ccelom) which connects with the nephridia by 

 nephrostomes. 



9. There is always a single ventricle arid, according to the 

 number of respiratory organs, one, two, or four auricles. 



10. The alimentary canal is well developed; the liver large; 

 salivary glands usually present. In most there is a pharynx or 

 buccal mass with radula and jaws. 



11. A veliger stage is common in development. 



12. The Mollusca are divided according to the respiratory organs 

 and appendages of the body into five classes: (1) Amphineura; (2) 

 Acephala; (3) Scaphopoda; (4) Gasteropoda; (5) Cephalopoda. 



13. The AMPHINEURA have an extremely simple nervous sys- 

 tem in which the three pairs of typical ganglia are replaced by 

 nerve tracts. 



14. The ACEPHALA, or Lamellibranchia, lack head and ceph- 

 alic appendages. 



15. They are bilaterally symmetrical and have paired organs: 

 mantle folds, bivalve shell, nephridia, and gonads. 



16. In many Acephala (Asiphonia) the mantle folds are com- 

 pletely separated ventrally. 



17. In the Siphonata the lower edges of the mantle are united, 

 leaving three openings: (1) in front for the foot; (2) behind and 

 below, the branchial siphon for the ingress of w r ater and nourish- 

 ment; (3) behind and above, the anal or excurrent siphon for the 

 water used by the gills and the faeces. 



18. There are two pairs of gills, which may be comb-like (true 

 ctenidia), filiform, or most commonly lamellar. 



19. Correspondingly the heart has two auricles; the unpaired 

 ventricle is usually traversed by the rectum. 



20. The foot is a compressed muscular mass frequently pro- 

 vided with a byssus gland. 



21. The shell consists of cuticular, prismatic layer and nacreous 

 layer. It is closed by two adductors and opened by an elastic 

 ligament. 



22. Some Acephals (Protoconcha) are very primitive in their 

 gill and hinge structure; others (Heteroconcha) are more highly 

 developed. 



