556 P. CHALMERS MITCHELL. 
cesophageal grooves could be made out, and if these are present 
they must be exceedingly obscure. 
The Mesenteries.—The mesenteries of the first order 
and those of the second order form twelve pairs of complete 
mesenteries. There is no anatomical distinction between 
these orders, nor between the tentacles (the twelve of the inner 
row) belonging to them. I examined one pair of directive 
mesenteries, which of course belonged to the mesenteries of the 
first order. In the directives the longitudinal muscles were, 
as usual, on the side of the mesentery turned towards the exo- 
cele. ‘The presence of the second pair of directive mesenteries 
I did not verify, as I wished to preserve as much as possible of 
the only specimen. 
There are twelve pairs of mesenteries of the third order. On 
the oral disc these reach the stomodeum, and extend about 
halfway down it. They have as broad an insertion into the 
pedal disc as the mesenteries of the first and second orders. 
The twenty-four pairs of mesenteries of the fourth order do 
not reach the stomodezum even on the oral disc. Down the 
column they extend as a narrow band, with a few longitudinal 
fibres, and on the pedal disc they are represented by radial 
ridges. The meseuteries are devoid of any apertures. 
Muscles.—The transverse muscles of the septa are in all 
cases weak. But the longitudinal muscles are exceedingly 
well developed, and form a series of pleated and stout bands, 
protruding from the surfaces of the mesenteries of the first, 
second, and third orders. In fig. 6 the arrangement of the 
muscles on the endoccelic face of a complete mesentery is 
represented. The longitudinal muscle on the part of the 
mesentery in the oral disc is not well developed. <A few fibres 
pass along it towards the tentacular edge, but there is no 
special muscular insertion at the base of the tentacle. The 
great mass of the muscle is inserted into the upper surface of 
the oral disc vertically above the pedal disc. In the narrower 
part of the mesentery, between the stomodzeum and the top of 
the column, the muscle stands out as a very thick mass. 
Lower down two parts are obvious. There is a narrower and 
