312 THE BEAKED SPINDLE-SHELL. 



CEPHALOPHORA. 



WE now take leave of these highly developed mollusks, and pass to other forms where 

 the organization is not nearly so perfect, and where the habits are either so commonplace as to 

 be devoid of general interest, or the animals so shy that they never can be seen performing any 

 act which is likely to attract the attention of an unprofessed naturalist. 



It is an enormously large group, containing all the snails, whether terrestrial, aquatic, or 

 marine, the whelks, limpets, and similar animals not so familiarly known. Many species are 

 much used as food, while others are of great service in the arts, furnishing employment to 

 many hundreds of workmen. As the shell of these creatures consists of one piece or valve only 

 they are sometimes termed univalves, in contradistinction to the oysters, muscles, scallops, and 

 similar shells, which are termed bivalves, in allusion to their double shell. 



The larger number of mollusks are divided between the class which embraces the bivalve 

 shells and the present, the Cephalophora, or head -bearers ; the former are collectively termed 

 Acephala, or headless. The present class naturally take rank next after the Cephalopods, 

 so-called because the feet are arranged around the head, and both rank higher than the Ace- 

 phala, the headess, for the reason that they are more like the higher forms of life ; that is, 

 they are symmetrical ; have heads with a pair of eyes. 



The term Gasteropods has heretofore been used to designate this order, from the fact that 

 they crawl upon a flat disc, which was likened to a stomach, hence stomach-footed. 



The animals embraced in this order have what is called a lingual ribbon, or tongue, which 

 consists of a band of chitine, a peculiar substance which is characteristic of the skins of insects. 

 This is called an Odontophore, or tooth-bearer. It is attached to the floor of the mouth, and 

 lies free at one end, and bears on its upper surface numbers of hard, tooth-like processes. 

 When in use it is moved by muscles, and drawn over cartilages ; a rasping motion brings the 

 hard teeth into contact with any substance taken into the mouth for food. 



The mouth of these animals is situated on the under side of the head, and is armed by 

 variously situated jaws or plates of the hard glutinous character. Classification has been 

 greatly aided by the examination of these lingual ribbons. The symmetry of the typical 

 Cephalophoras is lost in the largest number by conforming to the shapes of the external parts 

 their shells. The cavity of the mouth communicates with an sesophagus, which sometimes 

 dilates and forms a crop, and then the stomach follows, from which the intestine arises. 



The circulatory system is well developed ; Dentalium being an exception, having no heart. 

 One auricle and one ventricle is usually present. The blood is colorless, the corpuscles of 

 which is nucleated. In both Acephala and the present class, the heart receives the blood from 

 the gills and forces it over the body. 



Respiration is by gills or by pulmonary organs, lamellar in form, and by plume-like 

 branchiae. The nervous system differs in the various groups ; ganglia or knots of nervous 

 matter arranged about the anterior parts around the " swallow," for example serve the func- 

 tions of brain. Organs of hearing are present ; eyes are generally so, and usually two i 

 number, situated upon the head, or some projecting appendages, called tentacles. The eyes 

 are singularly like those of vertebrates. The sexes are separate in some and in others com- 

 bined in one individual. Most of these forms lay eggs. In a few the young are produced 

 living, the eggs being hatched within the parent. 



Classification of these forms is yet in a most unsettled condition ; further study is required 

 for the determination of points of importance. Consequently provisional arrangement is all 

 that the student can look for. This should, of course, be understood, as learners are apt to 

 receive the mischievous idea that classification is fixed. 



Our first example of the gasteropods is the BEAKED SPINDLE-SHELL, so called from the 

 rather distant resemblance which its long and pointed form bears to a spindle, and the 

 elongated beak-like process which is seen pointing downwards to the ground as the animal 

 walks along. In the family to which this mollusk belongs, the lip of the shell is always 

 extended and deeply notched. 



