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 metastoma, 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 (¢f. 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 Ammoccetes 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 paleostracan as from the side of the vertebrate, for it signifies 
that a dwindling process has taken place in the foremost of the 
original prosomatic appendages—the chelicerze and the endognaths ; 
while, 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 Eurypterus and other cases 
