MR. R. OWEN ON THE ANATOMY OF CLAVAGELLA. 271 
out of water. It must serve, however, to keep up a communication between the chamber 
and its inhabitant ; and it is seen that the chamber has always a communication with 
neighbouring cavities in the rock by means of the calcareous tubuli!, the formation of 
which is determined by the proximity of those cavities. When, therefore, the Clavagella, 
by a sudden contraction of its adductor muscles, kas forcibly expelled the branchial 
currents from the siphon, as was observed to take place by Mr. Stutchbury, the space 
between the free valve and the walls of the chamber would be simultaneously filled, 
either by water rushing in through the tubuli, or forced out from the branchial cavity 
through the small anterior orifice of the mantle. 
The outer dermoid layer of the mantle is extremely thin, and where it does not line 
the valves it is mottled with minute dark spots, less numerous than those on the skin 
of Cephalopods, and presenting a glandular appearance under the microscope. The 
muscular layer, after forming the siphon and its retractors, is confined to the anterior 
part of the mantle, where it swells into a thick convex mass of interlaced and chiefly 
transverse fibres, attached to the valves along the sinuous submarginal depression above 
mentioned, and forming, I should suppose, one of the principal instruments in the work 
of excavation. No fibres could be detected in other parts of the mantle ; nor could any 
longitudinally radiating muscles be expected in a mantle which had no lobes to be re- 
tracted. 
The siphon, in the contracted state which it presented in the specimen, formed a 
slightly compressed cylindrical tube, half an inch in length, and the same in the long 
diameter. It is traversed longitudinally by the branchial and anal canals, which are sepa- 
rated from each other by a muscular septum, extending to the end of the siphon, beyond 
which the two tubes do not separately extend outwards ; and in this respect Clavagella 
agrees with Gastrochena and Aspergillum. The muscular parietes of the siphon were 2 lines 
in thickness ; the septum separating the branchial and anal canals was 1 line in thick- 
ness ; the diameter of each canal about 1 line: the inner extremity both of the anal and 
respiratory tube is provided with a valvular fold. Their terminations are beset with short 
papille. ‘The retractor muscles attach the siphon to the posterior adductor on one side, 
and to the anterior extremity of the oval mass of muscular fibres above mentioned on 
the other, leaving an intermediate space on both sides the body, which exposes part of 
the gills and labial tentacles. The muscular mass which bounds the anterior part of the 
animal’s body is of an oval form, 1 inch 3 lines in length, 8 lines in breadth, and varying 
in thickness from 2 to 3 lines: it is smooth and convex externally, and hollowed out 
within to lodge the viscera at the base of the foot, for the passage of which it leaves 
the small orifice above mentioned. The margins attached to the valves are more 
or less irregular; that which is affixed to the loose valve is the broadest, being at 
the ventral extremity 3 lines in breadth ; it may here be regarded as a third adductor. 
Posteriorly it is coutinued into the small adductor muscle. This muscle is marked 
1 e. Fig. 9. 
