28 W. G. RIDEWOOD. 



shield is about 2j 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 . . -PA mm. 



Total antero-posterior diameter .... l'23mm. 



Maximum width of shield . . . . . 1 • 1 mm. 



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 liuccal shield is thick and firm, and 

 stains with l;)orax 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, r.l., 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 nalTOw 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 l)y analogy with that of Rhahdopleura (which has been studied in the 

 living state), l)ut from the various positions in which the shield may be found in 

 the adults, and more particularly in the buds, of the species of Ceplialodiscus now 

 under consideration. It 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). 



