28 W. G. RIDEWOOD. 
shield is about 24 or 2% times the distance from the red line to the posterior edge 
(text-fig. 9, C). The average measurements taken from a number of well-expanded 
shields are as follows :— 
Centre of red line to front edge of shield. . 89 mm. 
Centre of red line to hind edge of shield. . 184 mm. 
Total antero-posterior diameter . : ; . 1°23 mm. 
Maximum width of shield . i ‘ : . 1: 1mm. 
The shield is attached to the body by its middle part only, and tears off 
readily. The tearing away of the shield lays bare a pair of fairly large holes, 
situated in front of the mouth and behind the bases of the plumes, and leading 
into the two collar cavities. The cavity of the shield (proboscis cavity) has the 
form of a cleft between the two layers of the shield, and is traversed by muscle 
fibres and connective tissue strands which have prominent nuclei along their 
course. The cavity opens dorsally among the bases of the plumes by a pair 
of small canals (proboscis pores) situated to the right and left sides of the 
pericardium. 
The middle part of the ventral face of the buccal shield is thick and firm, and 
stains with borax carmine more deeply than any other part of the body. A few 
scattered yellow-brown cells may be found in this part, but they are not crowded as 
they are at the edges of the shield. The outer face of the buccal shield is ciliated, 
but not the dorsal surface. 
The red line that is so conspicuous a feature in the buccal shield owes its 
existence to crowded granular cells of uniform character and bright red colour. 
The mass of red cells extends through the whole thickness of the ventral layer 
of the shield (text-fig. 10, 7./., p. 34), and the red line shows equally well on this 
face of the shield and on that face (dorsal) against which the mouth opens, for the 
cavity of the shield is but a narrow cleft in the region of the red line, and the 
oral layer of the shield is thin and transparent. 
The buccal shield is clearly an extremely mobile structure, a conclusion arrived 
at not merely by analogy with that of Rhabdopleura (which has been studied in the 
living state), but from the various positions in which the shield may be found in 
the adults, and more particularly in the buds, of the species of Cephalodiscus now 
under consideration. Jt is no unusual thing to find the buccal shield of an adult 
twisted on its stalk through 90 degrees, so that the anterior and posterior edges 
are right and left, and the curved red line is antero-posteriorly placed instead of 
being transverse to the length of the body. In half-grown buds the posterior lobe 
of the buccal shield is frequently bent outward at a right-angle or even at a smaller 
angle to the rest of the shield (see fig. 65, plate 7). In both buds and full-grown 
polypides the posterior lobe of the shield is thinner than the anterior lobe. In buds 
the shield is relatively narrower than in the adult (see text-fig. 9, A and B, 
and pp. 47-48). 
