396 MOLL use A. 



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

 organs are connected almost always occur: a, cerebral ganglia 

 and eyes; /j, 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 coelom) which connects with the nephridia by 

 nephrostomes. 



9. There is always a single ventricle and, 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 phar3Tix or 

 buccal mass with radula and jaws. 



11. A veliger stage is common in development. 



13. The Mollusca are divided according to the respiratory organs 

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

 Acephala; (3) Scaphopoda; (4-) Gasterojioda ; (5) Cephalopoda. 



1.3. The Amphineura have an extremely simjjle 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 jjaired organs: 

 mantle folds, bivalve shell, nephridia, and gonads. 



16. In many Ace^ihala {Asijjlionia) the mantle folds are com- 

 pletely separated ventrally. 



17. In the Sipho7iata the lower edges of the mantle are united, 

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

 below% the branchial siphon for the ingress of water 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 loot is a comjjressed 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 {rroioconclid) are very primitive in their 

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

 develojjed. 



