Generalia is 



elements produce differences in quality and appearance of the shells 

 — viz. opaque, porcelanous, polished. 



A rudimentary shell is present when the young mollusk leaves 

 the egg capsule, or is developed soon thereafter. This nucleus rapidly 

 increases its size by the addition of new shelly matter but is gen- 

 erally lacking in features which characterize older shells. Sculpture 

 is formed and color and pattern are added by the secreting edge 

 of the mantle, and any injury to this soft organ may result in some 

 deformity of the shell and partial loss of pattern and color. Alter- 

 nating periods of growth and inactivity are often defined by an axial 

 ridge or varix which represents the outer lip of a previous aperture in 

 univalve shells, and in lines or ridges of growth parallel to the 

 margin of bivalve shells. All irregularities about the apertures and 

 markings of shells are produced by corresponding irregularities in the 

 edge of the mantle, and remain as sculptural features of the shell 

 as new growth proceeds. Immature shells often differ from adult 

 specimens of the same species; juvenile shells are usually thin, have 

 thin lips and are lacking in the color and finished sculpture that orna- 

 ment the adult shell. These differences are sometimes so marked that 

 the identity of young shells has been unrecognized and independent 

 specific rank given them. 



Actual attachment of the mollusk to its shell is effected by 

 means of strong muscles. In gasteropods, one powerful muscle is 

 firmly fixed to the columella near the apex of the shell; in the pelecy- 

 pods one or two muscles are attached at opposite points in the two 

 valves. Contraction of these muscles keeps the valves tightly closed. 



Color is present only on the surface of shells and is believed to 

 be influenced by food and exposure to light. Shells from warm and 

 shallow waters are usually more brightly colored than shells of species 

 inhabiting the colder depths, and in most fixed bivalves the upper, 

 exposed valve is the more highly colored. 



Mollusks have well-developed systems for nutrition, respiration, 

 circulation, excretion, and reproduction — all under the control of a 

 nervous system. Many of them have organs of special sense; in 

 some instances even a structure analogous to the ear, which keeps 

 the animal in touch with its surroundings and aids in maintaining 

 its equilibrium. Many mollusks are sensitive to light and respond 

 actively to any change in its intensity; some possess compound eyes 



