MOLLUSCA. GASTEROPODA 235 



said to be imperforate, when instead there is an umbilicus 

 it is perforate. The opening of the umbilicus sometimes 

 becomes partly filled up with a shelly growth, known as 

 callus. The animal is attached to the columella by means 

 of a muscle, the contraction of which enables it to withdraw 

 completely into the shell ; but, when not retracted, the 

 coiled visceral mass only is covered by the shell. 



Usually the cavity of the gasteropod shell is continuous 

 from the apex to the aperture, but in a few cases partitions 

 are thrown across the earlier parts of the shell (fig. 98), 

 forming chambers which remain empty. The form of the 

 aperture varies considerably in different genera and is of 

 great importance in classification ; in shape, it may be 

 circular, oval, elongate, oblong, etc. Its margin is termed 

 the peristome : the outer part forms the outer lip (L), the 

 inner part (that next the columella) the inner lip. As 

 the gasteropod crawls along, the shell is carried on the 

 dorsal surface of its body with the apex directed backward 

 and upward, and the aperture downward ; consequently 

 the part of the aperture farthest from the apex is anterior, 

 the opposite (nearest the apex) is posterior. Sometimes, 

 as in Natica, there is no break in the peristome, and it is 

 then said to be entire or kolostomatous ; in other cases the 

 anterior border is notched or produced into a tube (ac) in 

 which the incurrent siphon is placed, and these forms are 

 said to be siphonostomatous ; sometimes there is also at the 

 posterior border another canal {pc), in which the excurrent 

 or anal siphon is placed. The outer lip may be thin and 

 sharp, or thickened. Sometimes it is curved outwards, and 

 is then said to be reflected] or it is curved inwards — inflected. 

 Its margin may be even, or crenulated, or produced into 

 processes. 



Many genera have a calcareous or horny plate, known 



