Qatty Marine Lahoratorij , St. Andrews. 45 



pinnules, would seem to cori-espond with the appearances. 

 The branchial apparatus of such forms would thus iu 

 their movements appear to have not only muscular aid, but 

 the important inHueuce of the coelomic fluid, so that the 

 ciliary action of the pinnules and filaments would materially 

 aid respiration as well as conduce to alimentation. 



Opercular Stalk. — The opercular stalk ai'ises 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. 

 I'he 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 >ide 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 rhouT- 

 ])oidal, 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 

 tissu'^ more distinct, whilst the modified hypoderm, which is 

 almost fibioid in section, maintains nearly an equal thickness 

 all round. Tiie base of the cone — that is, the dorsal (nlge — 

 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 

 fiattened externally or dorsally, 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 'ne- 

 comes thinner, and an elongate-ovoid and apparently muscular 

 area stretches from lateral projection to lateral projection, a 

 groove iu which the blood vessel lies (1*1, ^'. fig, 30) occurring 

 ventrally. The muscular fibres seem to pass to the calcareous 

 regiou 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 iu 

 sections of different examples, a feature due perhaps to 

 recently reproduced organs {cf. PI, VI. figs. 32 & 33j and 

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

 muscle and pseudo-chordoid tissue are both present. Tie 

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



