allied to Rhabdopleura. 341 
lamella, and in ordinary preparations are wholly covered by the 
disk. They are circular or irregularly rounded, of various 
shades of reddish brown, with occasionally a tinge of violet. 
In transverse section the eyes show a pale centre or a large 
clear globule, with an external ring of dark brownish cellular 
pigment. They lie over the ovary, and in dissection often 
remain attached to the eggs. They would appear to be 
mainly of service to the animal during the protrusion of the 
buccal disk and plumes. This form thus shows in the adult 
state what Prot. Vogt and Dr. Barrois found in the embryonic 
Loxosoma; for in the latter the double disk-like organ is con- 
nected with two eye-spots. 
The great buceal disk forms a thin plate with two slight 
and generally bilaterally arranged elevations in the centre 
anteriorly, and 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 pig- 
ment-grains, which are densest in the centre of the curve, 
and shade off gradually on each side; while a very conspi- 
cuous and well-defined 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 that in 
front) occurs. The two bands just mentioned form when 
complete a somewhat flattened ring. In many specimens, 
however, the brownish pigment is entirely removed by the 
spirit, leaving only the well-defined reddish posterior band. 
In transverse section the external or ventral surface of the 
disk is formed of finely granular and rather firm hypodermic 
tissue, closely marked with vertical strie. As we pass deeper 
(z. e. towards the dorsal surface) there is a tendency to form 
cells; while near the dorsal margin ovoid granular cells are 
very evident. These gland-cells are tinged in this region of 
a brownish hue by transmitted light. A basement-layer, 
possibly with a few muscular fibres, intervenes between the 
thin median hypoderm and the free parts of the disk on the 
dorsal aspect. The siructure of this organ accords in most 
respects with the careful researches of Prof. Allman on the 
organ in Rhabdopleura*. It seems chiefly to be a glandular 
hypodermic structure with secerning powers of great activity, 
probably in relation to the remarkable ccencecium. 
This disk differs from that in Rhabdopleura by its much 
greater size and its thickness. In the form just mentioned 
the organ is more nearly allied to the truncated and thickened 
opercular process of certain annelids ; while in Cephalodiscus, 
* Journ. Linn. Soc. Zool, vol. xiv. p. 585. 
