MOLLUSCS. 291 



ment does not exist in nature, there being numerous inter-relations between the different 

 groups. With this uncertainty it matters but little what classification we adopt, 

 though that which follows seems to the writer to best represent the present state of 

 our knowledge. All authors admit that the Scaphopoda are the lowest, while the 

 position of the Pteropoda is very uncertain, one of the latest writers including them 

 among the Cephalopoda. 



Sub-Class I. — Scaphopoda. 



The tooth shells, as they are commonly called, are few in number, but their pecu- 

 liarities have caused them to be regarded with considerable interest. They are of all 

 the Odontophora the most closely related to the Acephals. The shell is very long, 

 tapers slightly, and is either straight or curved like the tusk of an elephant, and is 

 open at both ends. The animal is attached to the shell near the smaller end, while 

 from the larger it protrudes a large number 

 of long and slender tentacles which are used 

 in obtaining food and as prehensible organs. 

 These tentacles arise far within the shell, 

 from a muscular ring surrounding the body. fig. 323.-2)<!«ta;mm removed from the shell: /.foot; 



o & "^ ff, genital organ; i, liver; m, mantle; «, kidney. 



In advance of this ridge, but still within the 



shell, is a cup, at the bottom of which lies the mouth. There are no eyes, no heart, 

 while the head is very rudimentary. The foot is large, three-lobed, and protrudes 

 from the larger end of the shell. The mantle is relatively very large and, like the 

 shell, is open at both ends. 



The mouth is armed with a lingual ribbon, and the alimentary canal, which is con- 

 voluted, terminates behind the origin of the foot. The liver is large and two-branched, 

 and the sexes are distinct. There are no special respiratory organs. We have already 

 spoken of the nervous system. 



The development of J)entaUum, which has been studied by Lacaze-Duthiers, is 

 very peculiar. After the formation of a morula, the body of the embryo becomes 

 surrounded with a number of rings of cilia, while at the anterior end is a tuft of 

 longer cilia. Gradually the anterior end becomes flattened into a disc, the edges of 

 which are ciliated, while the posterior part develops the mantle. This mantle secretes 

 the shell, which at first is open, but finally the edges of the mantle and the shell 

 unite, producing the tubular form found in the adult. 



The tooth shells are divided into two families (by Sars accorded a higher rank), 

 founded on characters of the shell and foot. In the first (Dentaliid^) the margin of 

 the smaller end is entire, or has a medium ventral slit, while the foot is three-lobed ; 

 in the Siphonopoda the foot bears a circular disc, the edges of which are armed with 

 papilljE, while the posterior end of the shell is either entire or with numerous 

 notches. 



The species are found in all seas and live buried in the sand, the smaller end of the 

 shell protruding, and through this the water necessary for res- 

 piratory purposes is drawn. The large foot is employed in bur- 

 ^^°' tols~tMth-"S ^"^ rowing, while the tentacles, which are ciliated, are employed in 

 capturing food, which consists of Foraminifera and other minute 

 animals. About one hundred species are known, represented on our coasts by De9v- 

 talium occidentale, Mitalis striolata and three or four other species. The largest 



