PHYLUM MOLLUSCA 



289 



Radula Vy.- 



Figure 178. Radula of a gastropod. Left, diagram of longitudinal section through anterior end 

 of a snail. Right, dorsal view of a few teeth from radula of a snail; the one on extreme left is 

 a central tooth. The radula is pressed against food and moved rapidly back and forth, rasping 

 off small particles. 



radula (Fig. 178), exists usually in the 

 mouth cavity or pharynx of all mollusks, 

 except the bivalves; it consists of rows of 

 chitinous teeth which tear up the food by 

 being drawn across it. Respiration takes 

 place primarily in the gills and in the man- 

 tle. Most fresh-water and land snails (pul- 

 monale gastropods) take air into the vas- 

 cularized mantle cavity, which thus serves 

 the purpose of a lung; or, they breathe 

 cutaneously. 



The sexes are usually separate, though 

 certain groups are hermaphroditic. The 

 number of eggs that develop in some mol- 

 lusks is very great; for example, nearly 500 

 million in the oyster in a single season. In 

 all such cases, the eggs when laid are sub- 

 jected to the dangers of the ocean currents 

 and numerous enemies. They also pass 

 through a metamorphosis after hatching. 

 Other mollusks lay very few eggs, for exam- 

 ple, Lymnaea, 20 to 100 and Mesodon, 40 

 to 100. 



The development of the eggs of most 

 mollusks includes a trochophore larval stage 

 (Fig. 179), which later develops into a 

 veliger larva, so called because of the pres- 

 ence of a band of cilia, the velum, in front 

 of the mouth. The velum is an organ of 

 locomotion and is somewhat responsible 

 for the dispersion of the species. However, 

 the major factor in the dispersal of most 

 marine bivalves is oceanic currents. Con- 



siderable importance is attached to the 

 presence of a trochophore in the develop- 

 mental history of certain mollusks, and 

 many embryologists are inclined to consider 

 this stage an indication of the ancestral 

 condition. According to this view, the mol- 

 lusks and annelids, which pass through a 

 trochophore stage in their ontogeny, have 

 been derived from a common ancestor. 



A newly found, primitive, 

 deep-sea mollusk 



The Danish "Galatheae Expedition" 

 dredging off the v^^est coast of Costa Rica 

 (9°23'N., 89°32'W.) found among the 

 many animals collected some extraordinar}^ 

 unidentified deep-sea mollusks. The 10 liv- 

 ing specimens were dredged up from a depth 

 of about 11,778 feet, or a little over two 

 miles. This new limpet-like mollusk was re- 

 ported by H. Lemche in Februar}', 1957. 

 Many zoologists regard the finding of Neopi- 

 lina galatheae as a far more important dis- 

 covery than that of the living coelacanth, 

 found a few years ago off the coast of 

 Africa. 



Neopilina is a living representative of the 

 class Monoplacophora. The class name was 

 originally proposed to cover a group of ex- 

 tinct primitive paleozoic mollusks. Now it 

 includes a living mollusk, which is radically 

 different from all other mollusks in that it 



