THE PROSOMATIC SEGMENTS OF AMMOCGTES 301 
belongs to the same segment as the lower lip is the fact that in 
addition to the tubular muscles a single ordinary striated muscle is 
found in the velum which, like the muscles of the lower lip, is 
innervated by this same mandibular nerve. 
This muscle is attached laterally to the muco-cartilage of the 
metastomal bar (ska) at its junction with the muco-cartilage of the 
lower lip, and spreads out into a number of strands which are 
attached at intervals along the whole length of the free anterior 
edge of the velum. It is the only non-tubular muscle belonging 
to the velum, and by its contraction it draws the anterior portions 
of the velar folds apart from each other, and so opens the slit 
between them, through which the food and mud must pass. Clearly 
from its position it does not belong to the original tergo-coxal group 
of muscles as do those of the lower lip; it must have been one of the 
intrinsic muscles of the metastoma itself. 
This anterior portion of the velar folds affords yet another 
striking hint of the correctness of my comparison of the lower lip 
seoment of Ammoccetes with the chilaria of Limulus or the metas- 
toma of Eurypterus ; for the most dorsal anterior portion, which at 
its attachment possesses a wedge of muco-cartilage, forms a separate, 
well-defined, rounded basal projection marked Ser.in Fig. 115, and B 
in the accompanying Fig. 120. This is that part of the velar folds 
which comes together in the middle line and closes the entrance into 
the respiratory chamber. The epithelial surface here is most striking 
and suggestive, for it is markedly serrated, being covered with a 
large number of closely-set projections or serre. The serration of 
the surface here is of so marked a character that Langerhans con- 
sidered this part of the velar folds to act as a masticating organ, 
grinding and rasping the food and mud which passed through the 
narrow slit. In fact, Langerhans supposed that this portion of the 
velum acted in a manner closely resembling the action of the gnatho- 
bases of the prosomatic appendages in Limulus or the Eurypteride. 
This suggestion of Langerhans is surely most significant, con- 
sidering that this somewhat separate portion of the velum, to which 
he assigns such a function, is in the very place where the gnathite 
portion of the metastomal appendages would have been situated if it 
were true that the lower lip and anterior portion of the velum of 
Ammoccetes were derived from the metastoma, 
In addition to this marked serrated edge the whole surface of 
