TWO NEW TYPES OP ACTINIARIA. 
147 
at any point. Their arrangement, beginning with the directive 
pair is as follows : 1, 3, 2 , 3 , 1, 3, 2 , 3, 1, 2 , 1, 3*, 2 , 1, 3, 2 , 3 , 
1, 3, 2 , 3. Over each pair of mesenteries is placed a tentacle, 
with the exception of the pair marked above with an asterisk, 
the position of which suggests that it was more recently deve- 
loped than the rest. Muscles are present on both faces of the 
mesenteries ; those on the outer (ectocoelic) aspect are the pro- 
tractors, and are recognisable in transverse section only at the 
upper part of the mesentery (fig. 5). Their function is to assist 
the ectodermal longitudinal muscles of the oral surface and 
stomodseum in the expansion of the animal. Those on the 
inner (entoccelic) aspect are the retractors, by which a general 
contraction is effected, and the stomodaeum pulled upwards 
and inwards ; they are indicated in fig. 3 by faint lines on the 
mesentery. 
The free edge of the mesentery is not thrown into much 
contortion, and for the most part bears the normal form of 
filament (fig. 7). Besides this, however, occurs on most 
mesenteries a structure, which I can neither describe nor 
figure with any accuracy, owing to scantiness of material and 
imperfect preservation. In transverse section of the polyp it 
sometimes presents the appearance represented in fig. 8, but 
more often appears as a solid swelling on the edge of, or in the 
centre of, the mesentery. Above and below it the ordinary 
form of filament often occurs. A number of these structures 
were extremely obvious when the larger specimen was stained 
and cleared (fig. 1), they then presented a gastrula-like 
appearance. 
In the smallest specimen, eleven pairs of mesenteries were 
present, of which the rather larger six pairs are primary, the 
remaining five secondary. No directive pair is present. The 
number of tentacles (fourteen) would seem to indicate that, as 
is often the case, the addition of new cycles of mesenteries is 
to a certain extent preceded by multiplication of the tentacles. 
The mesenteries essentially agree with those of the larger 
specimen, except for the fact that they do not exhibit the 
peculiar form of (?) filament. The contortion of the free edge 
