54 



In accordance with variations in these morphological indices, the shell 

 may be round, ovate, trigonal-ellipsoid, and so forth. It may also be equi- or 

 inequilateral. In equilateral shells the anterior margin of the valve is symmetric 

 or nearly symmetric to the posterior margin; in inequilateral shells it is asym- 

 metric. In the larvae of some species the left and right valves of the shell may 

 be different in shape and size. Shells with equal-sized valves are called equivalve 

 and those with unequal valves, inequivalve. 



3. Structure of hinge : Length of provinculum, number of teeth on each 

 valve, their shape and size, and also the number and shape of special teeth, and 

 the structure of the lateral hinge system — these are of taxonomic importance. 



4. Presence, disposition, and structure of ligament : In position, the liga- 

 ment may be outer or inner. The inner ligament may be anterior, median or 

 posterior relative to the hinge line. In transverse section it may be rounded, 

 rectangular, square, or oval. 



5. Sculpture of larval shell. 



6. Thickness of valves and their transparency. 



7. Presence of eyes : Visible as dark pigment spots, distinct on both sides 

 of the shell, in live as well as mounted larvae. 



8. Color of larva and presence of characteristic pigmentation of internal 

 organs preserved in mounted material. 



In identification of live larvae such features may be taken into consider- 

 ation: size and shape of velum; presence or absence of apical ciliary tuft and 

 length of its cilia; shape of intestinal spiral; position and structure of gill 

 filaments; shape of depression leading to duct of byssus gland; position and 

 number of retractors of the foot and velum, and so on (Lebour, 1938; Allen, 

 1961; D'Asaro, 1967; Chanley and Andrews, 1971; Chanley and Castagna, 

 1971; Dinamani, 1973; Culliney, 1974; Turner and Boyle, 1974; Culliney et 

 al., 1975). However, one must exercise caution in using these parameters 

 because the internal structure in developing larvae even at the same stage is 

 highly variable, making interspecific comparison difficult. The fairly usefiil 

 index for identification of larvae of different species is their behavior, since 

 each "tracks" differently while swimming, as does the pediveliger while creep- 

 ing (Turner, 1975). 



In view of the fact that in general practice the identification of larvae is 

 based on mounted specimens, and that larval tissue becomes deformed during 

 mounting, the principal method of identification relies on features character- 

 izing the larval shell. However, the most apparent taxonomic character of shell 

 structure — structure of the hinge system of the larva — is difficult to ascer- 

 tain during examination of large plankton samples. Considerable time is re- 



