92 



Certain larval types are also mentioned in the following discussion, most 

 of which are associated with molluscan development. It is within the 

 mollusks that one of the greatest varieties of larval development is found; 

 as most of the other invertebrate groups have a pelagic larval development, 

 swimming about in the surface or surrounding waters for a considerable 

 length of time (most echinoderms), or several successive larval stages, 

 which may be free-swimming or passing part of their existence as parasites 

 (most crustaceans, although not malacostracans). Lamellibranch species 

 may have planktotrophic or pelagic larval development, where the veliger 

 or trochophore larvae may spend anywhere from a few hours to several 

 weeks swimming about in the water before settling. Others may have a 

 lecithitrophic development, where the larva is furnished with considerable 

 food in its egg and may either have a direct development from the egg 

 without a pelagic existence, or else the pelagic existence is very short 

 and the larvae does not feed before settling. Other lamellibranchs may 

 even have brood protection, sheltering their young until they are ready 

 to take up a direct development, without a trochophore stage. Gastropods 

 have an even more varied larval development, ranging from those species 

 which have trochophore larvae with pelagic stages lasting more than a 

 month to those that lay relatively few large eggs from which young snails 

 hatch and crawl away, or those that even bear living young in the image 

 of the adult. An extensive discussion on reproduction and larval develop- 

 ment of invertebrates can be found in Thorson (1950). 



An examination of existing literature on present-day marine benthic 

 communities, as well as many fossil assemblages, led to the conclusion 

 that a basic organization of animals living together results from an inter- 

 dependence of biological factors and an overall dependence upon the 

 physical environment (see Hesse, Allee and Schmidt, 1951 and Thorson, 

 1950). The environmental extremes not only influences the physiological 

 processes of the dominant or characteristic animals, but it may exert a 

 control on the distribution and abundance of food, especially plankton 

 and organic detritus. Thorson (1957) has proposed that parallel bottom 

 communities, characterized by the same or closely related genera of 

 dominants, exist wherever environmental conditions are similar. Perhaps 

 these parallel communities are not so parallel in generic compositions as 

 they are in the biological organization of the community as suggested in 

 part by Hesse, Allee and Schmidt (1951). One type of community found 

 in all waters with the same set of environmental conditions may be char- 

 acterized by particular percentages of feeding types and dominated by 

 animals with certain kinds of reproduction or larval development. It is 



