710 



ZOOLOGY 



SECT. 



trochophore is, however, distinguished from the corresponding stage 

 in Worms by the presence of the shell-gland, which soon secretes 

 a delicate impaired shell. The prostomial region grows out into a 

 thickened retractile rim bearing the pre-oral circlet of cilia, and 

 called the velum (Fig. 594 vel.) : the larva at this stage is 

 distinguished as a vcligcr a very characteristic molluscan phase 

 of development. The shell soon becomes bivalved and extends 

 ventrally on each side, paired processes of the dorsal region of the 

 body accompanying it and forming the mantle-lobes. A projection 

 grows out from the ventral surface, between mouth and anus, and 

 forms the foot (Fig. 595, /), and on the sides of the body the gill- 

 filaments (k) arise as a row of delicate processes, at first simple 



H 



sd^ 



A 



B. 



^Q 



P ~> 



Sk 



-5 1 , 



ff 



n.--- 



f. 



FIG. 595. Two embryos of Cyclas. a. anus; by. byssus-gland ; /. foot; <j. gonad ; /.-, gill; 

 m. mouth ; m+l. stomach and "liver " ; uir. edge of mantle ; n. kidney ; p. pericardium ; s'. 

 unpaired shell ; s". rudiment of paired shell ; sd. shell-gland ; i-d. gullet ; vel. velar area. 

 (From Korschelt and Heider.) 



but afterwards becoming bent upon themselves so as to assume a 

 V-shape. Eyes are often present in the larva at the base of the 

 velum. 



General Remarks. Although none of the Pelecypoda are 

 microscopic, they present a considerable range in size, from the 

 minute Nucula, about 4 mm. long, to the Giant Clam (Tridacna 

 gigcts) of the Indian and Pacific islands, which is sometimes 60 cm. 

 (two feet) in length and 500 pounds in weight. 



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 



