306 THE ORIGIN OF VERTEBRATES 



lip is the continuation forwards of the original ventral surface of such 

 an animal as Limulus or a member of the scorpion group, where there 

 is no inetastoma, and corresponds to the endostoma, as Holm calls it, 

 of Eurypterus. This termination of the ventral surface in all these 

 animals is made up of two parts : (1) Of sternites composing the true 

 median ventral surface of the body, called by Lankester the pro- 

 and meso-sternites ; and (2) of the sterno-coxal processes of the fore- 

 most prosomatic appendages, called in the case of Limulus gnathites, 

 because they are the main agents in triturating the food previously 

 to its passage into the mouth. In Limulus, a conjoined pro-meso- 

 sternite forms the median ventral wall to which the sterno-coxal 

 processes are attached on each side, and in Phrynus and Mygale a 

 well-marked pro-sternite and meso-sternite are present, forming the 

 posterior limit of the olfactory opening. In Buthus and the true 

 scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th pro- 

 somatic appendages take part in surrounding the olfactory tubular 

 passage ; in Thelyphonus only the processes of the 2nd pair of pro- 

 somatic appendages play such a part, the pro-sternite not being 

 present (cf. Fig. 97). 



Seeing, then, what a large share the sterno-coxal processes of one 

 or more of these prosomatic appendages plays in the formation of 

 this endostoma, and seeing also that the nerve which supplies the 

 upper lip-muscles in Ammocoetes is the same as that supplying the 

 tentacles which are attached to the upper lip, it appears to me more 

 probable than not that the muscles in question are the vestiges of 

 the sterno-coxal muscles. These muscles differ markedly in their 

 attachments from the muscles of the lower lip, for whereas the latter 

 resemble the tergo-coxal group in their extreme dorsal attachment, 

 the former resemble the sterno-coxal group in their attachment to 

 what corresponds to the endostoma. 



This interpretation of the meaning of the transformation process 

 is in accordance with all the previous evidence both from the side 

 of the palaeostracan as from the side of the vertebrate, for it signifies 

 that a dwindling process has taken place in the foremost of the 

 origiual prosomatic appendages — the chelicerie and the endognaths ; 

 Avhile, on the contrary, the ectognath and the metastoma have con- 

 tinued to increase in importance right into the vertebrate stage. 

 This process is simply a continuation of what was already going on 

 in the invertebrate stage, for whereas in Enrypterus and other cases 



