352 DK W. CARMICHAEL M'LNTOSH ON THE STRUCTURE OF THE 



revolving with great velocity. Such motions are doubtless due to the ciliation of 

 the chamber. On making a transverse section of the living animal (0. alba), I 

 have seen the inner margin of the digestive cavity cause motion in the surround- 

 ing particles, but the cilia were indistinct, and the appearances very different from 

 the richly ciliated tube of Borlasia, or its own oesophageal portion anteriorly. It 

 is thus much more feebly ciliated than the others. 



In the walls of this complex cavity are a vast series of gland-cells, which, 

 with M. Van Beneben, I consider as having some analogy with the liver of the 

 higher forms, notwithstanding the adverse opinion of Prof. Keferstein, who, how- 

 ever, probably refers more particularly to the Porlasians. Microscopically the 

 cells consist of a delicate membrane containing a number of fatty globules (Plate 

 X. fig. 6), the average size of the cell being «wth of an inch. Under pressure, 

 and when highly magnified (700 diam.), it is seen to consist of a number of 

 granular fatty bodies (Plate X. fig. 7). After extrusion from a living specimen 

 into salt water, a remarkable motion occasionally ensues in the contents of the 

 cell before breaking up, a condition which causes the observer to fancy the entire 

 organ ciliated. The contained bodies jerk about within the cell, and soon a 

 number of very minute granules appear, having burst from the larger bodies, in 

 which their presence is indicated by obscure markings. The peculiar motions would 

 seem to be due to the action of the water, and ultimately the minute contained 

 bodies are all set free. The various appearances of the bodies from the cells are 

 shown in Plate X. fig. 8, some being granular, others presenting faint con- 

 centric lines like starch-globules (though probably fatty), while three oil-globules 

 are indicated on the right. The deep port-wine oil-globule is somewhat sparingly 

 scattered throughout the wall of the tract, the yellowish red being abundant, and 

 the pale globule still more plentiful. These cells have a similar structure in 

 Tetrastemma, and often escape under pressure posteriorly. The quantity of deep 

 yellow oil in this organ in T. alga? is unusually great. The foregoing glandular 

 structure undergoes partial absorption at the period of reproductive activity, so 

 that after spawning the animal is much flattened ; but by-and-by it regains its 

 plumpness, and often becomes of a greyish hue, apparently from the increased 

 development of this tissue, which is exuded as a pale, salmon-coloured, semi-fluid 

 substance on rupture of the body-w r all. In 0. gracilis the posterior division of 

 the digestive system has a somewhat regularly ramified arrangement, when 

 viewed from the ventral surface, and this is especially evident some time after 

 spawning, when the animal has regained its condition. The colour of the region 

 is of a deep green by transmitted light, whereas the oesophageal division is 

 brownish. The lamellae of this region in 0. pulchra form simple tapering papilla? 

 under pressure. In Polia involuta, V. Ben., the cavity is greatly developed, 

 both as regards the rest of the body and its individual structures ; and it also 

 presents a firmer and more consistent aspect than usual on transverse section. 



