Gatty Marine Laboratory^ St. Andrews. 45 



pinnules, would seem to correspond with the appearances. 

 The branchial apparatus of such forms would thus in 

 their movements appear to have not only muscular aid, but 

 the important influence of the ccelomic fluid, so that the 

 ciliary action of the pinnules and filaments would materially 

 aid respiration a^ well as conduce to alimentation. 



Opercular Stalk. — The opercular stalk arises as a process 

 of the basal region of the branchial apparatus immediately 

 in front of the brain, the tissues of one side gradually 

 projecting (PI. IV. fig. 20), then being nipped off as an 

 independent process surrounded by the cuticle, the modified 

 hypoderm as a considerable coat all round, and a central 

 area more or less muscular at first, with numerous nuclei. 

 The base of tl e organ occupies at first more than half the 

 dorsal outline, but, as it separates and the median fissure 

 deepens, the other side increases in bulk. The external 

 fold of the cuticle bends inward, the hypodermic cells 

 curving round the central area (PI. IV. fig. 22) and soon 

 the stalk is free. Its outline in section is somewhat rhoin- 

 boidal, and much smaller than it is distally. At this level the 

 thoracic jacket or collar is fixed by a broad isthmus to the 

 region below the gullet. Then the stalk becomes conical in 

 section, and the blood-vessel in the centre of the muscular 

 tissue more distinct, whilst the modified hypoderm, which is 

 almost fibroid in section, maintains nearly an equal thickness 

 all round. The base of the cone — that is, the dorsal edge — 

 by-and-by lengthens by a transverse projection at each side, 

 so that it resembles a cocked hat in section (PI. VI. fig. 32), 

 the projecting edges having the thickest hypoderm from the 

 approximation of the two layers separated by a line, the 

 central pseudo-chordoid and muscular areas with the vessel 

 remaining as before. The opercular stalk at this level is 

 flattened externally or dorsalty, convex ventrally, and its 

 cuticle is dense. A differentiation of the central region now 

 takes place, for the outer or dorsal edge of the hypoderm he- 

 comes thinneiyuid an elongate-ovoid and apparently muscular 

 area stretches from lateral projection to lateral projection, a 

 groove in which the blood vessel lies (PI. V. fig. 30) occurring 

 ventrally. The muscular fibres seem to pass to the calcareous 

 region of the operculum — namely, to the tip of the stalk. 

 They are well developed in the region of the lateral ridg< s. 

 The appearance of the parts seems to vary considerably in 

 sections of different examples, a feature due perhaps to 

 recently reproduced organs (cf. PI. VI. figs. 32 & 33) and 

 to obliquity in section, for in some cases (PL VI. fig. 33) 

 muscle and pseudo-chordoid tissue are both present. The 

 reticulations of the next (more distal) area are larger and 



