196 MORPHOLOGY OF INVERTEBRATE TYPES 



Digestive system. The mouth, as already mentioned, is sit- 

 uated in the centre of the oral surface. It is surrounded by the 

 circular lip and armed with five teeth placed interradially and 

 belonging to a complicated apparatus known under the name of 

 Aristotle's lantern. The hollow axis of this lantern is formed by 

 the pharynx while the body of the lantern which has the shape 

 of a pyramid with a pentagonal base is composed of five com- 

 plicated calcareous parts or jaws and as many groups of muscles. 

 When isolated each calcareous jaw appears in the shape of a 

 triangular pyramid. The middle portion of the tooth is inclosed 

 between the two halves of an ossicle called the alveolus. The 

 elastic free upper end of the tooth is curved over the base of the 

 pyramid and inclosed in a pouch of the oral sinus. The horn- 

 like processes of the alveoli serving for the attachment of the 

 protractors are termed the epiphysis and, though fused with the 

 alveoli, are in reality separate ossicles. Radiating from the 

 middle of the lantern at its base are five ossicles articulated to 

 the alveoli and called rotulce. Below the rotulae and also radial 

 in position are five compasses or F-shaped ossicles, called by that 

 name on account of their two diverging ligaments. These long 

 and thin ligaments arise side by side from the head or distal 

 enlargement of the compass and are attached to the peristomial 

 edge of the two adambulacral plates on each side adjoining the 

 radius to which the ossicle belongs. The muscular apparatus 

 of the lantern is very complicated. It consists of seven sets of 

 muscles aggregating no fewer than sixty individual muscles. 

 Of these we will consider the following: (i) Five inter pyramidal 

 (or interalveolar) muscles. These short muscles are attached to 

 the adjoining radial surfaces of the alveoli. They hold the al- 

 veoli together and close the teeth. (2) Five pairs of protractors. 

 They are attached to the epiphysis and the peristomial edge of 

 the test and run to the inside of and parallel to the compass- 

 ligaments. (3) Five pairs of retractors attached to the external 

 surface of the alveoli near the teeth and to the auricles. (4) Five 

 muscles binding together the compasses and forming the di- 



