242 THE ORIGIN OF VERTEBRATES 



the foremost mesosomatic segments by numbers (8-13). I have 

 placed the four endognaths and the nerves going to them close 

 together, and made them small, mere tentacles, in recognition of the 

 character of these appendages in Eurypterus, and have indicated the 

 position and size of the large ectognath, with its separate nerve, 

 by (6). If among the ancient Eurypterus-like forms, which were 

 living at the time when vertebrates first appeared, there were some 

 in which the ectognaths also had dwindled to a pair of tentacles, 

 then such animals would possess a prosomatic chamber formed by 

 a metastoma or accessory lip, within which were situated five pairs 

 of short tactile appendages or tentacles. If the vertebrate were 

 derived from such an animal, then the trigeminal nerve, as the 

 representative of these prosomatic appendage-nerves, ought to be 

 found to supply the muscles of this accessory lip and of these five 

 pairs of tentacles in the lowest vertebrate. 



This prosomatic or oral chamber, as it might be called, was limited 

 posteriorly by the fused metastoma (7) and operculum (8), so that if 

 in the same imaginary animal one imagines that the gill-chambers, 

 instead of being separate, are united to form one large respiratory 

 chamber, then, in such an animal, a prosomatic oral chamber, in 

 which the prosomatic appendages worked, would be separated from 

 a mesosomatic respiratory chamber by a septum composed of the 

 conjoined basal portions of the mesosomatic operculum and the 

 prosomatic metastoma, as indicated in the diagram. In this septum 

 the nerves to the last prosomatic appendage (equivalent to the last 

 part of the trigeminal in the vertebrate) and to the first mesosomatic 

 (equivalent to the thyroid part of the facial) would run, as shown in 

 the figure, close together in the first part of their course, and would 

 separate when the ventral surface was reached, to pass headwards 

 and tailwards respectively. 



The Coxal Glands. 



One more characteristic of these appendages requires mention, 

 and that is the excretory glands situated at the base of the four 

 endognaths known as the coxal glands. These glands are the main 

 excretory organs in Limulus and the scorpions, and extend into the 

 basal segments or coxae of the four endognaths, not into those of the 

 ectognaths or the chilaria (or metastoma). Hence their name, coxal 



