PHYLUM MOLLUSCA 



661 



Many pelecypod shells are white or dull brown in colour, but 

 in several genera brilliant tints are the rule, the various species of 

 Scallop (Pecten} being specially remarkable in this respect. The 

 inner surface of the shell often exhibits beautiful iridescent tints, 

 noticeably in the so-called Pearl-oyster (Meleagrina) and the 

 Australian Trigonia. As far as is known, the colours are all what 

 are called " non-significant," i.e. are of no physiological or ethological 

 importance. In this connection the formation of pearls by some 

 species must be mentioned : they are deposits of nacre formed 

 round sand-grains or other foreign bodies, either between the 

 mantle and shell or in the soft parts. They are produced, amongst 

 other species, by the " Pearl-oyster " (Meleagrina margaritifera) 



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FIG. 558. Two embryos of Cyclas. a. anus ; by. byssus gland ; /. foot ; g. gonad ; I; gill ; 

 TO. mouth ; m+l. stomach and liver ; wr, edge of mantle ; n, kidney ; p. pericardium ; s'. un- 

 paired shell ; s". rudiment of paired shell ; sd, shell gland ; i-d, gullet ; ret. velar area. (From 

 Korschelt and Heider). 



and by the Pearl-mussel (Unio margaritifera). Some species, 

 such as the common boring Pholas. are phopphnresppnt. 



Most Pelecypc^^r"e~ : s^a^giBh'irr habit, progressing only by slow 

 contractions of the foot, and some are permanently fixed, during 

 adult life, by the byssus. The Scallops, however, swim freely by 

 clapping the valves together. The Cockles (Cardium), Trigonia, 

 etc., jump by sudden movements of the foot, and the Razor-fish 

 (Soleri) jerks itself forward by suddenly withdrawing its foot and 

 thus ejecting water through the siphons. The only parasitic 

 genus is Mntovalva, found in the gullet of a Holothurian. 



Pelecypoda are abundant both in fresh water and the sea ; the 

 marine forms are mainly littoral. None are pelagic or terrestrial. 

 They are very abundant in the fossil condition, occurring in all 

 formations from the Upper Cambrian upwards, and, owing to 

 their gregarious habits, frequently forming extensive deposits or 

 shell-beds. The oldest forms are all iso- or hetero-myarian the 



