PROSOMATIC SEGMENTS OF LIMULUS 241 
internally to it is disclosed, which, in his view, corresponds to the 
sternite between the bases of the pro-somatic appendages in Limu- 
lus, z.e. to the sternite called by Lankester, the pro-mesosternite (£nd., 
Fig. 103). This inner plate formed with the metastoma ((7) Fig. 
105) and the ectognaths (6) a chamber closed posteriorly, within 
which the bases of the ectognaths worked. In other words, the 
removal of the metastoma discloses in Eurypterus the true anterior 
ventral surface of the animal which corresponds to that of Limulus, 
or of the scorpion group, with its pro-mesosternite and laterally 
attached gnathites or sterno-coxal processes. To this inner plate or 
pro-mesosternite Holm gives the name of endostoma. 
To the anterior edge of the endostoma a thinner membrane is 
attached which passes inwards in the direction of the throat, and 
forms, therefore, the lower lip (Hyp., Fig. 105, B) of the passage of 
the mouth (olf. p.). This membrane bears upon its surface a tuft of 
hairs, which he thought were probably olfactory in function. Con- 
sequently, in his preliminary communication, he describes this lower 
lip as forming, in all probability, an olfactory organ; in his full 
communication he repudiates this suggestion, because he thinks it 
unlikely that such an organ would be situated within the mouth. I 
feel sure that if Holm had referred to Croneberg’s paper, and seen 
how the true mouth in all the scorpion group is situated at the base 
of an olfactory passage, he would have recognized that his first sug- 
gestion is in striking accordance with the nature of the entrance to 
the mouth in other scorpions. 
That Eurypterus also possessed a camerostome (cam.) seems to 
follow of necessity from its evident affinities both with Limulus 
and the scorpions. We see, in fact, that the mouth of these old sea- 
scorpions was formed after the fashion of Limulus, surrounded by 
masticatory organs in the shape of foot-jaws, and yet foreshadowed 
that of the scorpion, so that an ideal sagittal section of one of these 
old paleostracan forms would be obtained by the combination of 
actual sagittal sections through Limulus and a member of the scorpion 
group, with, at the same time, a due recognition of Holm’s researches. 
Such a section is represented in Fig. 105, B, in which I have drawn 
the central nervous system and its nerves, the median eyes (CZ), 
the olfactory organs (Cam.), the pharynx (Ph.), cesophagus (@s.), and 
alimentary canal (AJ.), but have not tried to indicate the lateral eyes. 
I have represented the prosomatic appendages by numbers (1-7), and 
R 
