146 
G. HERBERT FOWLER. 
pointed out ( f Chall. Rep. Zool. Actiniaria/ Supplt., p. 12), the 
occurrence of the latter muscle-layer on the “ mauerblatt 55 is 
confined to Corynactis and Cerianthus among the Anthozoa, 
but is characteristic of both hydriform polyps and Scyphistomse 
among the Hvdrozoa. As there is no reason to believe that 
Thaumactis is derived from an Actinian-like ancestor with a 
“ mauerblatt/ 5 the presence of this ectodermal longitudinal 
muscle is of considerable phylogenetic interest. Both sets of 
muscles are extremely weak, and consist merely of single 
parallel fibrils, which produce a slight unevenness of the 
mesogloea lamina. 
In two regions, however, of the wall of the body, these 
muscles attain to a more considerable development, namely, on 
the indrawn part of the oral surface, and on the stomodseum. 
On the ectodermal side of the mesoglosa in both these regions 
are developed longitudinal muscle-fibres, adhering to pleatings 
of the mesogloea (fig. 5), by which the expansion of the animal 
is doubtless effected. On the endodermal side of the invaginated 
part of the oral surface occurs a strong circular muscle, forming 
a true sphincter of the “ diffuse 55 type ; in the contracted condi- 
tion, this is continuous from the bases of the tentacles right on 
to the horizontal surface, where it passes into the general circular 
muscle; it is the chief muscle concerned in the invagination of 
the disc during retraction. The endodermal circular muscula- 
ture of the stomodseum is very slight, its function being 
merely to close the entrance into the coelenteron during 
digestion. The musculature of the tentacles has been noticed 
in connection with them. 
The mesenteries amounted in the largest polyp to twenty- 
one pairs, of which one pair only were directive mesenteries. 
Of the total number, six pairs (including the directive) are 
“ primary/ 5 and are attached along the whole length of the 
stomodseum; six are “ secondary/ 5 of which those four only 
which lie nearest to the directive meet the uppermost (in the 
expanded state) part of the stomodseum ; while the nine pairs 
of tertiary mesenteries are developed chiefly in the neighbour- 
hood of the directive pair, and do not touch the stomodseum 
