V. MOLLUSCA: SUMMARY OF IMPORTANT FACTS 347 



had simple septa. AMMOXITID/E, folded septa, largely mesozoic. Their 

 pertinence to the tetrabranchiates is assumed from the character of the shell. 



Order II. Dibranchia. 



With two nephridia, two gills, and two auricles; eight or ten arms with 

 suckers; highly organized eyes; shell rudimentary or absent. 



Sub Order I. DECAPODA. Ten arms, body with lateral fins. Rudimen- 

 tary shell usually present. SPIRULID^;, with internal chambered loose-coiled 

 shell. Spirula (fig. 353). OIGOPSIDA, with perforated cornea (p. 338) and two 

 oviducts. Ommastrephes*; Architcntkis,* the giant squid (p. 336). MYOPSIDA. 

 Oviduct single (left); cornea unperforated. Loligo* common squid; Rossia*; 

 Sepia, cuttle fish, furnishing the 'cuttle bone' fed to cage birds, and the pigment 

 sepia. Sub Order II. OCTOPODA. Eight arms, webbed at their base; shell 

 very rudimentary, sometimes fragmentary or wanting; oviducts paired. Ocro- 

 PODID^;, Octopus'^ (fig. 362), Alloposus.* ARGONAUTID.^, female with boat- 

 like shell (fig. 363), males much smaller and without shell. In Argonautidse 

 and PHILONEXID.E the hectocotylus separates. 



Summary of Important Facts. 



1. The MOLLUSCA are parenchymatous animals with reduced 

 coelom. They consist of head, visceral sac, mantle, and foot. 



2. The head bears eyes and tentacles. 



3. The foot is an unpaired muscular mass used in locomotion. 



4. The mantle bounds the mantle cavity which is connected with 

 respiration; it either functions as a lung or covers the gills (ctenidia). 

 It secretes the shell from its outer surface. 



5. Foot, head, mantle, and with the latter the shell, may be lost in 

 many groups. 



6. The molluscs agree in the nervous system. Three pairs of ganglia, 

 connected with three pairs of sense organs, occur: a, cerebral ganglia and 

 eyes; b, pedal ganglia and statocysts; c, visceral ganglia and osphradia 

 (olfactory). 



7. The heart is dorsal and arterial; it is enclosed in a pericardium 

 (reduced ccelom) which connects with the nephridia by nephrostomes. 



8. There is always a single ventricle and, according to the number 

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



9. 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. 



10. A veliger stage is common in development. 



11. The Mollusca are divided according to the respiratory organs 

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

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



12. The AMPHINEURA have an extremely simple nervous system 

 in which the ganglia are replaced by nerve tracts. 



