CHAPTER XII 

 PHYLUM MOLLUSCA 



The phylum Mollusca includes essentially bilaterally symmet- 

 rical, unsegmented Metazoa in which the coelom has been 

 reduced by invasion of connective tissue and of musculature 

 until only the pericardial cavity and the lumen of the gonads 

 represent its remains. The typical bilateral symmetry is 

 frequently lost or obscured through secondary coiling or torsion 

 of parts of the body. Head, visceral mass, foot, mantle, and 

 shell are characteristic structures, but in many instances one or 

 more of these are lacking and all are subject to great variability 

 in the various groups. A rasplike organ known as the radula is 

 associated with the mouth of most molluscs except the Acephala. 

 This is a bandlike membrane bearing numerous cross rows of 

 chitinous teeth for rasping off food material. 



A modified trochophore, termed the veliger, is a larval form 

 common to all of the major groups of molluscs, though in the 

 modified development of some forms it has been suppressed. 

 A larval organ, known as the shell gland, is typical though not 

 always retained. Reproduction is exclusively sexual. 



The usual means of locomotion is by crawling on an unpaired 

 foot. Powers of locomotion are lost in some sessile forms and 

 in still others swimming is made possible through modifications 

 of various structures. The mantle, when present, secretes the 

 shell and bounds a cavity which may either contain the gills 

 or function directly as a lung. The central nervous system is 

 typically composed of three pairs of ganglia of which each pair 

 is associated with a special sensory organ. The cerebral ganglia 

 communicate with the eyes, the pedal ganglia with the statocysts, 

 and the visceral ganglia with the osphradia or olfactory organs. 



In size, Mollusca range from minute individuals barely visible 

 to the eye to some of the largest known invertebrates. One of 

 the giant squids reaches a length of 18 feet. 



The Mollusca have continued as important and conspicuous 

 components through all the geological periods in which animal 

 remains have been found. 



184 



