250 



THE ANIMAL KINGDOM 



Lens 



■Retina 



Oplic n^-rve. 

 Figure 14.6. Section through the eye of a whelk. (After Helger.) 



each tentacle is a small eye. The eye is well developed and probably 

 forms images. It is spherical (Fig. 14.6), lined with a retina, a layer of 

 light-sensitive cells, and contains a large globular lens. The outer sur- 

 face, except where the light comes through, is pigmented to screen out 

 extraneous light. 



In Busycon sexes are separate. A single gonad lies in an upper 

 whorl of the body, between the digestive glands. From the ovary an 

 oviduct passes down through the mantle, opening near its anterior edge 

 to the right of the anus. Near its end the oviduct is surrounded by a 

 yellow shell gland. A sperm duct from the testis opens on the right 

 side into the posterior limit of the mantle cavity. From there a ciliated 

 sperm groove leads across the body (Fig. 14.5) to the base of a large 

 penis just behind the right tentacle. The groove continues along the 

 penis to its tip. Fertilization is internal. 



The eggs are laid in cases secreted by the shell gland, a dozen eggs 

 to the case. The cases are arranged in a row along a connecting strand 

 (Fig. 14.7) that is attached to the bottom. The young pass through all of 

 their larval stages within the cases, emerging as small whelks. The cases 

 are tough, lasting long after the young have emerged, and are often 



Mi 



/ *^I*^9^K 



Figure 14.7. The egg case of Busycon. (Photo by Hugh Spencer.) 



