206 INTRODUCTION. 
and longitudinal, unstriped, nuclear fibres, and is the ori- 
gin of the especial retractor muscles of different organs. 
Chapter iv. — of the digestive apparatus. 
Lijiax. The orifice of the mouth is bounded by a 
pair of contractile lips, is situated at the anterior part of 
the head, and opens into the cavity of the buccal body. 
When the latter is retracted by its peculiar muscle, the 
oral orifice becomes lengthened into a canal by the in- 
version of a portion of the external integument. 
The buccal body is an irregularly oval-shaped, muscu- 
lar organ, resembling in appearance a gizzard, and con- 
tains -within it the masticatory apparatus. Just within 
the upper lip, attached to the entrance of the buccal 
body, is the dental plate, a crescentic, corneous lamina, 
used for cutting the food. Its anterior face is convex, 
and presents several vertical ridges. Into its upper 
convex edge, a band of muscular fibres is inserted, by the 
contraction of which the inferior, concave, cutting edge 
13 advanced beyond the line of the upper. The middle 
of the cutting edge is extended into a short, conical 
toothlet. This plate is brought into view when the ani- 
mal is eating, by the advancement of the buccal body. 
The floor of the cavity is occupied with a gouge-shaped, 
muscular tongue, the tip and upper surface of which are 
free, and are covered by a corneous lamina studded with 
a great number of conical dentures, with the points pro- 
jecting backwards, arranged in transverse rows. These 
