318 SIR SIDNEY F. HARMEE ON 
III. Internal Avicularia. 
Levinsen (1909, p. 139, pi. 2. figs. 7Z>, d, e, g, j, k) has described the- 
occasional replacement o£ an ordinary frontal aviculariuirij in Menipea 
roborata, by an avicularium which grows into the body-cavity ; and in one 
o£ his letters quoted in Sect. V. (p. 336) he refers to the presence of internal 
avicularia in M. marionensis. He makes no comment on this extraordinary 
occurrence ; but however improbable it may seem, both on morphological 
and on physiological grounds, there is no doubt of the accuracy of the 
description. I have found these internal avicularia, not only in the two 
species indicated by Levinsen, but also in certain other species of the same 
genus. 
The internal avicularia of M. marionensis are represented in three of my 
figures. PL 19. fig. 44, a basal view, shows one of these structures (i. av.y 
in each of the zocecia D and E. The avicularium is a more or less cylindrical 
structure situated on the inner side of the frontal wall, and projecting basally 
into the body-cavity. The palatal surface is on the basal side, there is a 
normal beaked rostrum, with which is connected an acute, triangular 
mandible. The material is not in the best condition, and it has the 
appearance of having been allowed to dry. The polypides are, however,, 
perfectly recognisable, and there seems to be clear evidence that the mandible 
opens and closes in the body-cavity, and in actual contact with the tissues of 
the polypide. The arrangement seems an inconvenient one, and I can suggest 
no explanation of its purpose. There is no evidence whatever that the 
avicularium lies in a cavity invaginated from the frontal side, and I cannot 
escape from the coiaclusion that the mandible actually moves in the body- 
cavity. Four of these internal avicularia are shown, in basal view, in PI. 17. 
fig. 22, in the zocecia C, D, B, and K. The base of the cylindrical avicularium,. 
i.av. (PL 19. fig. 43), is not raised above the general level of the frontal wall, 
and from it originate the occlusor muscles, which are seen in frontal view, of 
the mandible. The avicularium has in fact an inverted position, and has been 
developed on the inner side of the wall of the zooecium, instead of on its 
outer side. 
The account given by Levinsen of the internal avicularia of Menijjea 
roborata is not easy to understand ; and a renewed study of these remarkable 
structures was required. There is no doubt that, as stated by Levinsen, the 
internal avicularium replaces an ordinary frontal avicularium. I do not find 
them always present, as he states, when an external avicularium is absent, 
and I have not found them in the marginal rows. They may usually be 
found without difiiculty in some of the zocecia of the submarginal rows, and 
occasionally in other zocecia further removed from the margin. They seem 
to be always wanting on the distal side of an ovicell, where two external 
avicularia are constantly present, directed obliquely distally, and thus reversed 
