PROSOMATIC SEGMENTS OF LIMULUS 24 1 



internally to it is disclosed, which, in his view, corresponds to the 

 sternite between the bases of the pro-somatic appendages in Lirnu- 

 lus, i.e.. to the sternite called by Lankester, the pro-mesosternite (End., 

 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 (ram.) 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 pakeostracan 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 (C.E.), 

 the olfactory organs (Cam.), the pharynx (Ph.), oesophagus (ces.), and 

 alimentary canal (Al.), but have not tried to indicate the lateral eyes. 

 I have represented the prosomatic appendages by numbers (1-7), and 



1; 



