PROSOMATIC SEGMENTS OF LIMULUS 243 
glands; and, seeing the importance of the excretory function, it is 
jikely enough that they would remain, even when the appendages 
themselves had dwindled away. With the concentration and 
dwindling of the endognaths these coxal glands would also be con- 
centrated, so that in the diagram (Fig. 105) they would rightly be 
grouped together in the position indicated (cow. gi.). 
Such a diagram indicates the position of all the important organs 
of the head-region except the special organs for taste and hearing. 
These, for the sake of convenience, I propose to take separately, in 
order at this stage of my argument not to overburden the simplicity 
of the comparison I desire to make with too much unavoidable detail. 
THE PROSOMATIC REGION OF AMMOCCETES. 
Let us now compare this diagram with that of the corresponding 
region in Ammoccetes and see whether or no any points of similarity 
exist. 
With respect to this region, as in so many other instances already 
mentioned, Ammoccetes occupies an almost unique position among 
vertebrates, for the region supplied by the trigeminal nerve—the 
prosomatic region — consists of a large oral chamber which was 
separated from the respiratory chamber in the very young stage by 
a septum which is subsequently broken through, and so the two 
chambers communicate. 
This chamber is. bounded by the lower lip ventrally, the upper 
lip and trabecular region dorsally, and the remains of the septum or 
velum laterally and posteriorly. It contains a number of tentacles 
arranged in pairs within the chamber so as to form a sieve-like fringe 
inside the circular mouth; of these, the ventral pair are large, fused 
together, and attached to the lower lip. 
All the muscles belonging to this oral chamber are of the 
visceral type, and are innervated by the trigeminal nerve. In 
accordance with the evidence obtained up to this point this means 
that such an oral chamber was formed by the prosomatic appendages 
of the invertebrate ancestor, similarly to the oral chamber just figured 
for Eurypterus. 
This chamber in the full-grown Ammoccetes is not only open to 
the respiratory chamber, but is bounded by the large upper lip (U.Z., 
Fig. 106, D). On the dorsal surface of this region, in front of the 
