FRONT-GILLED GROUP 3339 



FRONT-GILLED GROUP Order PROSOBRANCHIATA 



The Prosobranchs, which include the majority of marine gastropods and a few 

 groups of terrestrial forms, are always contained within or protected by a shell, 

 and organized for crawling. The branchial or pulmonary cavity is in advance of the 

 heart, and the auricle of the latter in front of the ventricle. This important anatom- 

 ical characteristic, which is expressed or referred to in the term Prosobranchiata, 

 can best be understood by observing the figure on p. 3338 representing the male 

 of one of the periwinkles (Littorina). Here the respiratory organs are lodged in 

 a chamber formed by the mantle behind the head; the gills are variable in the 

 different suborders. In some forms, with nonspiral shells, they are double and 

 symmetrical, but in others, and these are the majority, where the shell assumes a 

 spiral form, the gill is usually single. Some of the Prosobranchs have horny jaws, 

 but others are without; most are furnished with a radula, but this is sometimes 

 absent. Characteristics derived from this organ have been largely employed in 

 the classifications of the suborders. The sexes are separate; the head is distinct, 

 and furnished with a pair of tentacles, sometimes having the eyes at the ends, 

 but mostly at or near their base. An operculum, which is unknown among the 

 Opisthobranchs and Pulmonates, except in the case of Amphibola, is developed in 

 the majority of Prosobranchs, but the number of genera in which this appendage 

 is absent is considerable. It assumes very different forms, and its size is very 

 variable in proportion to the dimension of the aperture of the shell. In some 

 genera it is as large as the aperture of the shell (Ampullaria, Natica, Bithynia), 

 while in others it only partly closes it; and in other cases it is so small as to be of 

 no use for closing the shell. It is mostly horny, but in a few instances is more or 

 less thickened and strengthened with a shelly or calcareous layer. The horny oper- 

 cula are mostly flattened, but in a few cases are thickened, or conical (Torima, 

 Ccelopoma}. The shells of Prosobranchs comprise some of the most beautiful, and 

 most prized by collectors. The order may be separated into three main divisions, 

 namely, Pectinibranchiata, Heteropoda or Nucleobranchiata, and Scutibranchiata, 

 severally characterized by differences, as the names imply, in the breathing organ, 

 coupled with other characteristics in connection with the genital organs, radula, etc. 



SUBORDER Pectinibranchiata 



The mollusks included in this suborder have the body twisted, and contained 

 in a spirally-coiled shell. They are chiefly aquatic, and mostly marine, but a few 

 families are strictly terrestrial. The gills are comb-like, that is, consist of plates 

 arranged side by side and attached to a common stem, like the teeth of a comb to 

 the thickened back. They are situated in a vaulted chamber, formed by the 

 mantle over the neck, which is open in front for the admission of air or water. 

 The Pectinibranchs have been subdivided into several sections, founded upon 

 modifications of the radula or lingual ribbon. The following are the most im- 

 portant in the arrangement adopted by Paul Fischer, namely, Toxoglossa, 



