OLIGOTREMA PSAMMITES. 245 



network with rather open meshes. The arrangement of these 

 muscle-bands in Oligotrema is exceptional in that the circular 

 bands are external and the longitudinal bauds internal. 



Towards the posterior end of the body the musculature 

 diminishes and eveutually dies out altogether, but it is 

 strongly developed round the atrial siphon. The last-named 

 structure is a long, narrow tube formed by the walls of the 

 ectoplerome, and in a surface view, such as is shown in fig. 3, 

 it mig'ht easily be taken for a continuation of the intestine. 

 The muscles in this siphonal tube are arranged, like those of 

 the bodj^-wall, as an external layer of circular aud an 

 internal layer of longitudinal muscle-bands, which here are 

 relatively very numerous and form a rectangular network, 

 with very fine meshes. 



Immediately below the branchial aperture the circular 

 muscle-bands are gathered together to form a strong 

 sphincter surrounding the mouth. This branchial sphincter 

 (fig. 4, sp/(.) lies above a riug oE thickened tissue surrounding 

 the mouth, and embraces the branchial siphon. It is chiefly 

 made up of a number of flat, concentric, muscular bands, which 

 extend outwards over the bases of the tentacular arms in 

 such a manner as to give a hexagonal figure to the sphincter. 

 These circular muscle-bands are to be regarded as the con- 

 strictors of the branchial orifice, and they are crossed by a 

 number of much finer radial bands or dilators of the branchial 

 orifice, whose inner ends are inserted on the thickened ring 

 of tissue mentioned above. The same ring of tissue serves 

 for the attachment of the longitudinal muscles of the body- 

 wall and the flexor muscles of the arms, to be described 

 further on. The muscle-bands of the dilators, as well as 

 those of the constrictors, branch, aud the branches of the 

 latter inosculate with one another in a complicated manner, 

 especially at the bases of the arms (fig. 5, S2)h.). At 

 the base of each arm a bundle of muscle-fibres is given off 

 from the outermost band of the sphincter and branches freely 

 in the substance of the arm, the branches looping round in the 

 manner shown in fig. 5, aud uniting again to form a number 



