692 ZOOLOGY SECT. 



passes into a stage in which it closely resembles the trochophore 

 of Chaetopods (Fig. 603), having a prototroch and an apical tuft 

 in the middle of the prostomium. There is also an ectodermal 

 thickening on the prostomium which becomes the cerebral ganglion, 

 and a similar ventral thickening which gives rise to the pedal 

 ganglion and corresponds with the rudiment of the ventral nerve- 

 cord in Polychaeta. The pelecypod trochophore is, however, dis- 

 tinguished from the corresponding stage in Worms by the presence 

 of the shell-gland, which soon secretes an unpaired shell. The 

 prostomial region grows out into a thickened retractile rim bearing 

 the pre-oral circlet of cilia, and called the velum (Fig. 604, Vel.) : 

 the larva at this stage is distinguished as a veliger a very charac- 

 teristic mulluscan 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. 605, /), and on the sides 

 of the body the gill-filaments (k) arise as a row of fine processes, 

 at first simple 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 Nitcula, about 4 mm. long, to the Giant Clam (Tridacna 

 gigas) 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 

 species must be mentioned : they are deposits of nacre formed 

 usually round encysted parasitic worms, either between the mantle 

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

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

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

 as the common boring Pholas. are phosphorescent. 



Most Pelecypoda are sluggish in habit, progressing only by slow 

 contractions of the foot, and some are permanently fixed during 

 adult life by the byssus, or are only able to change their position 

 after throwing off the byssus, which becomes replaced by a new 

 one. The Scallops, however, swim freely by clapping the valves 

 together. The Cockles (Cardium), Trigonia, &c., jump by sudden 

 movements of the foot, and the Razor-fish (Solen) jerks itself 



