PROSOMATIC SEGMENTS OF LIMULUS 243 



glands ; and, seeing the importance of the excretory function, it is 

 likely 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 (cox. gl.). 



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 Peosomatic Eegion 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 

 prosoinatic 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 

 iuside 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 fall-grown Ammoccetes is not only open to 

 the respiratory chamber, but is bounded by the large upper lip (U.L., 

 Fig. 106, D). On the dorsal surface of this region, in front of the 



