8 THE VOYAGE OF H.M.S. CHALLENGER. 



the pedicle is extended (PI. III. fig. 1) the ventral surface is nearly straight and con- 

 tinuous with the pedicle, which leaves the body posteriorly at the ventral edge, while, the 

 kidney-shaped mass of the body projects dorsally. The pedicle in contraction is shorter 

 than the body, is nearly cjdindrical, and terminates in a rounded end ; but in extreme 

 elongation its appearance is much more attenuate. It is marked ventrally by various 

 longitudinal striae from the muscular bands. 



The anterior region of the body curves somewhat suddenly downwards and back- 

 wards, and forms a flattened surface on which the great buccal disk or prse-oral lobe 

 rests. 



Buccal Disk. 



The great buccal disk (PI. II.; PL VI., fig. 2, bs) forms a thin plate with two 

 slight and generally bilaterally arranged elevations in the centre anteriorly, and is 

 divided into two regions by a notch at each side, the anterior moiety being the larger 

 and thicker. The surface of the latter is marked by an arch of brownish pigment-grains, 

 which are densest in the centre of the curve, and shade ofi" gradually on each side ; while 

 a very conspicuous and well-defined deep brownish-red band commences in the posterior 

 division at the notch, and runs with a backward curve to the opposite side. Between 

 this and the posterior margin a brownish pigment-belt — less developed than in front — 

 occurs. The two bands just mentioned form, when, completed, a somewhat flattened 

 ring. In many specimens, however, the brownish pigment has been entirely removed by 

 the spirit, leaving only the well-defined reddish band posteriorly. 



In intimate structure (PI. VI. figs. 2, 3, bs) ^ the disk is found to consist of a ventr3,l 

 plate, the superficial characters of which have been described, and a pedicle. In 

 section the former presents the features of a great hypodermic shield somewhat similar 

 in structure to the Nemertean or Annelidan skin, and the surface of which in life is 

 probably clothed with cilia, as indeed Professor G. 0. Sars found in Rhahdopleura, 

 though no distinct cuticular layer is visible in the preparations. This hypodermic tissue 

 is marked in section by vertical striae, which in the thick anterior central region assume 

 a somewhat radiate aspect. The free parts of the shield present dorsally a firm basement- 

 tissue under the cuticle, while ventrally, that is, on the secreting surface, the granular 

 glandular tissue terminates in a translucent smooth edge. As the pedicle is approached, 

 a narrow reticulated region appears within the basement-tissue strengthening the dorsal 

 region of the disk, and the inner as well as the outer wall of this region assumes a firm 

 structure — so as to resemble the basement-tissue. In the centre of the pedicle this 

 region is traversed by a radiate series of muscular fibres, which spring from the firm tissue, 

 constituting a kind of skeleton strengthening the basal part with which the plumes are 



' I am much indebted to my assistant, Mr. John Wilson, B.Sc, for making a series of finely stained sections of 

 Cephalodiscus. These have enabled me to determine features not fully seen in unstained sections made in cork by the 

 hand. 



