23 



IV. ORr,y\J:IZATION AND LATE DEVELOPMENT OF THE "SHIP-V/ORMS. 

 a. The Gonoral O;-/: '...i;-.at ion. 



In their form and general oi'f:;anization, the adult 

 ".'^'hip-worms" aro the most peculiar and striking of all the 

 Lamcllibranchs. Their newly-attached larvae have the form 

 and most of the organization of the larvae typical of most 

 of the ir.urine Lamellibranchs , v/hich are free-sv^imming dur- 

 ing their larval development. The transfomiation of the 

 typical larva into the highly specialized "Sh.ip-v/^orm" 

 takes place v;ith such great rapidity as to properly be 

 called a metamorphosis. 



The Larva and its i.ieta: cwphosis. - The general shape 

 of tlae newly-attachod larva (fig. 1) is that of a small 

 clam, v^itli equal shell valves. These latter are broader 

 than long, so that this Lamellibranch which has a more 

 elongated adult, has a larva more fore-shortened perhaps , 

 than any other form. The yoixngest specimens I have foixnd 

 were creeping over tlie surface of the wood by means of 

 their very active, tongue-shaped foot. The velum v.-as in 

 all cases retract^^d into the large velar cavity (v.c), a 

 large space in the anterior, dorsal part of the shell cav- 

 ity, v.vn in fig. 24, v/hich is a sagittal section 



