Chap. III. GENUS PLEUROBRACHIA. 243 



thread ^vlien this is fully expanded, and their varions other disiwsitions, their 

 frequent straight and apparently stiff elongation, and, still more, their sudden 

 bending even in acute angles. These motions are so diversified, and sometimes 

 so sudden, as to astonish even those familiar with the movements of these animals. 

 Having described above the position and changes of form of the digestive cavity, 

 I have now only to add, that its inner surfiice has not throughout the same 

 appearance, and is not uniformly flat. Near the aperture of the mouth, indeed, 

 it is smooth ; and, when the mouth is fully expanded, a broad funnel is opened, 

 leading directly into the digestive cavity, assuming, however, in its contractions, 

 very diversified forms, being at times perfectly circular, and at other times oblong, 

 oval, or even angular. The anterior and posterior angles of the mouth form fre- 

 quently a fold, or it assumes a linear shape, or a stellate form. The more the 

 mouth is open and spread out, the more easy is it to follow to a considerable 

 depth the tubes which extend vertically along the walls of the stomach or coeliac 

 cavity. The walls of this cavity present four folds, two of which are in the 

 direction of the fissure of the mouth, along the anterior and posterior walls of the 

 stomach, and two others at right angles Avith them along the middle of its broader 

 wall, in the plane of the transverse axis of the body. These four folds are lined 

 with brown cells, suggesting the idea of a rudimentary livei-, or, at least, that of 

 secreting cells aiding in the process of digestion. Towards the abactinal extremity 

 of the digestive sac, between those prominent folds, the walls of the digestive cavity 

 are lined with a vibrating epithelium, which is particularly active round the abacti- 

 nal opening of the sac, when this is fully open. This vibrating epithelium is con- 

 tinued upon the inner surfiice of the central chymiferous cavity, into which the 

 stomach pours its contents. 



If we now view this animal from the abactinal side, we find a variety of organs, 

 the structure and connections of which are not easily imderstood. Considering them 

 at first chiefly in their relations to one another, it will be seen that there is in 

 that region an elongated area, well circumscribed in its outlines, extending in a 

 longitudinal direction, in the same plane as the mouth, with a black speck in its 

 centre (PI. IP. Fig. 20). Towards the centre of this area eight narrow bands are 

 seen converging, and, in an oblique position to its axis, near the black speck, two 

 slight projections may be observed near the margin of the area. The black speck 

 in the centre rests upon a tubercle within, which is itself encircled by a fork of 

 the funnel. This organ, which is considered as an eye-speck by some anatomists, 

 and as an auditive sac by others, is a globidar or broad pyriform mass {d) of large, 

 highly refractile cells or vesicular bodies, altogether enclosed in a large, exceedingly 

 transparent, dome-like capsule {&'). The pyriform mass {8) is attached to the bot- 

 tom of its capsule {8') by its narrower end, and is constantly nodding or vibrating. 



