MOLLUSCA — GASTROPODS 



239 



wound whorls form an elongate cone whose base is the aperture 

 of the shell, and whose apex, the oldest portion of the cone, cor- 

 responds to the umbo of pelecypods. The axis of union of the 

 whorls is the columella (Fig. 103, co.). It forms the inner bound- 

 ary of the aperture called the columellar or inner lip in opposition 

 to the thin outer lip. The operculum is a small accessory shell, 

 a fiat oval plate, borne on the foot, which on the withdrawal 

 of the head and foot into the shell closes the aperture (Fig. 

 102, op.). 



■0. 



Fig. 104. — Busycon canalicidatus. A, portion of a string of egg capsules. Natural 

 size. B, three views of the young shell (4 mm. long) when ready to leave the cap- 

 sule by the opening 0. in Figure A. Top sketch shows view looking upon apex 

 of spire. The first or smooth whorl is called the protoconch {pr.). 



The sexes are distinct. The eggs are fertilized within the body 

 of the female and are laid in great masses together with abundant 

 nutritive material. In Busycon these masses of eggs and food 

 are in the form of a long series of disk-shaped parchment-like 

 capsules, like checkers on a string (Fig. 104). There are on 

 the average 70 capsules in each string and each capsule con- 

 tains about 50 embryos surrounded by much albuminous fluid 

 which serves as nourishment during the growth from eggs to 



