400 COMPARATIVE ANATOMY chap. 



E. The Mastieatopy Apparatus of the Eehinoidea. 

 (Aristotle's Lantern.) 



In all Eehinoidea, with the exception of the Spatangoida, and per- 

 haps of a few Holedypuida, the mouth, which lies at the centre of the 

 peristomal area, is armed Avith five hard and pointed, interradially 

 arranged teeth. These teeth are approximated or moA-ed apart by 

 means of a complicated masticatory or jaw apparatus lying within 

 the test, and resting on the peristome. This apparatus is known as 

 the lantern of Aristotle, and is of considerable size ; it is covered on 

 all sides by a closely applied integument, the lantern membrane, a 

 continuation of the peritoneum. The spaces Avithin the masticatory 

 apparatus are completely separated by this membrane from the spacious 

 liody cavity within the test. 



The masticatory apparatus resembles a pentagonal pyramid, the 

 base of which is directed upwards, i.e. projects into the cavity of the 

 test, while the tip, formed by the five teeth, lies in the mouth. Its 

 axis is traversed by the oesoj^hagus. It consists essentially of skeletal 

 pieces, muscles, and ligaments. 



a. The skeleton of the masticatory apparatus (Fig. 347) is 

 composed of twenty-five pieces (including the teeth), radially grouped 

 around the .oesophagus ; some of these pieces have received very un- 

 suitable names. There are five teeth, five pairs of jaws (alveoli), 

 five "sickles" (falces), and five radii or rotulse. The " sickles "' 

 may be named intermediate plates, and each pair of jaws forms a 

 " pyramid." 



The principal 2>art of the framework of tlie masticatory apparatus is formed liy 

 the five interradially placed pairs of jaws. These determine the conical or pyi'amidal 

 form of the whole framework. The two pieces of each pair are firmly connected with 

 one another on the outer side of the framework by a vertical iuterradial snture, and 

 together form a hollow triangular pyramid, the fifth part of the whole pyramidal 

 framework. Each single pyramid thus has one outer and two lateral surfaces. The 

 five single pyramids are in contact with one another along these lateral surfaces, 

 which lie radially to the axis of the whole framework. The edges along which the 

 lateral surfaces come in contact are the axial edges, i.e. those turned towards the oeso- 

 phagus. The suture, which divides each single pyramid into two halves or jaws, runs 

 down the outer surface, exactly halving it. The walls of each single hollow pyramid 

 are, however, not complete : (1) the two lateral surfaces do not quite meet along 

 their inner edges, but there is a slit left between them (Fig. 347, E) ; (2) the basal 

 wall (that turned upward) is wanting ; when the soft parts are removed an aperture 

 is found here, the foramen basale, which leads down into the cavity of the pyramid ; 

 (3) a large incision (foramen externum) is found^ at the base of the outer wall, and 

 is either confluent with the foramen basale or is separated from the latter by an 

 arch, the arcus. 



The single pyramids (or pairs of jaws) are the sup^iorts and carriers of tlie teeth. 

 Each tooth is a long, slender, and hard skeletal piece, curved, so that its convex side 

 faces outwards ; it traverses the cavity of the jiyramid, and projects beyond it at both 



