DEVELOPMENT OP GAgTBROPODS. 



521 



FiQ. 269. 



Palate of Buccinum undaium^ as 

 seen under polarized light. 



fill objects for the Polariscope, when they are mounted in Canada 

 balsam ; the form and arrangement of 

 the teeth being very strongly brought 

 out by it (Fig. 269), and a gorgeous 

 play of colors being exhibited when a 

 selenite plate is placed behind the ob- 

 ject, and the analyzing prism is made 

 to rotate. 



348. The stomachs, also, of many 

 Gasteropod MoUusks are furnished 

 with teeth, which are implanted on 

 their walls for the further reduction of 

 the food ; such teeth, very numerous 

 but of small size, and bearing a strong 

 resemblance to those of its tongue, are 

 found in the s.tomach of the common 

 Slug. In several marine Gasteropods, 

 however, especially Bulla, Scyllcea, and 

 Aplysia, the gasti'ic teeth are individu- 

 ally much larger, though less numerous, 

 and constitute a very efficient reducing 



apparatus, especially when combined, as they frequently are, 

 with a horny or calcareous deposit in the walls of the stomach, 

 which converts it into a " gizzard" for the trituration of the 

 substances that have been divided by the teeth. 



349. Development of G-asteropod MoUusks. — The history of em- 

 bryonic development may be studied with peculiar facility in 

 certain members of this class, and presents numerous pheno- 

 mena of great interest. The eggs (save among the terrestrial 

 species) are usually deposited in aggregate masses, each enclosed 

 in a common protective envelope. The nature of this envelope, 

 however, varies greatly : thus in the common Lymnceus stagnalis, 

 or "water-snail," of our ponds and ditches, it is nothing else 

 than a mass of soft jelly, about the size of a sixpence, in which 

 from 50 to 60 eggs are imbedded, and which is attached to the 

 leaves or stems of aquatic plants ; in the Buccinum undatum, or 

 common Whelk, it is a membranous case, connected with a con- 

 siderable number of similar cases by short stalks, so as to form 

 large globular masses, which may often be picked up on our 

 shores, especially between April and June ; in the Purpura 

 lapillus, or Rock-whelp, it is a little flask-shaped capsule, having 

 a firm horny wall, which is attached by a sort of foot to the 

 surface of rocks between the tide-marks, great numbers being 

 often thus found standing erect side by side; whilst in the 

 Nudihranchiate order generally (consisting of the Boris, Uolis, 

 and other " sea-slugs") it forms a long tube with a membranous 

 wall, in which immense numbers of eggs (even half a million or 

 more) are packed closely together in the midst of a jelly-like 

 substance, this tube being disposed in coils of various forms, 



