METRIDIUM 179 
Exercise 1. Draw the expanded animal, showing the column and 
the disc, with the mouth and the tentacles. 
Internal anatomy. Cut the animal into halves by a longitudinal 
incision passing at right angles to the mouth and from the dise 
to the foot. The mouth will be seen to open into a flattened 
tube with more or less corrugated walls, called the gullet. Note 
the formation of the siphonoglyphs. The gullet leads into the 
gastro-vascular space, which is the general internal cavity of the 
animal. 
The most prominent structures in this cavity are the mesenteries, 
which are longitudinal partitions extending from the outer wall 
of the body inward toward its center. These mesenteries will 
be seen to occur in pairs; six of these pairs, called the primary 
mesenteries, join the body-wall with the wall of the gullet. The 
pair at each angle of the gullet which enclose the siphonoglyphs 
between them are called the directives. Between the six pairs of 
primary mesenteries are secondary, tertiary, and quarternary pairs. 
The gastro-vascular space is thus divid'd into a large number 
of partially separated longitudinal chambers. 
Note carefully the structure of the free edges of the mesen- 
teries below the gullet. The thickened corrugated structure 
which forms the edge is the mesenterial filament; it contains 
digestive glands. From the base of the mesentery extend the 
acontia. The reproductive organs, the ‘testes and ovaries, are 
also located in the mesenteries, lying alongside the mesenterial 
filaments. 
Note carefully the position of the longitudinal muscle bands, 
one of which is present on the surface of each mesentery. It is 
by means of these muscles that the body is contracted. A circu- 
lar muscle in the disc closes the mouth by its contraction and aids 
in drawing in the tentacles. 
Exercise 2. Draw a semidiagrammatic view of the cut surface of 
the animal, showing these features. 
