TRANSACTIONS OF SECTION I. 965 
protuberance; then the modification of the median projections of the foremost 
ventral appendages—the pedipalpi—to form the hypostoma, in the corresponding 
position the upper lip or hood of Ammoccetes, which forms the hypostoma as far 
as the hypophysial tube or palzostoma is concerned, but an upper lip as far as the 
new mouth is concerned. The muscles of this upper lip belong all to the splanch- 
nic and not to the somatic group, and are innervated by the appropriate nerve of 
the prosomatic appendages, viz. the motor part of the Vth. Ventral to the pedi- 
palpi in Thelyphonus there is nothing, ventral to the corresponding lip in the 
Ammoceetes is the lower lip, and we have seen that, although such a structure is 
absent in the land scorpions of the present day, it was present in the sea scorpions 
of old time, was known as the metastoma, and is supposed to be a forward growth 
which started at the junction of the prosoma with the mesosoma. Precisely corre- 
sponding to this we see from Kuppfer that the lower lip of Ammoccetes is a forward 
growth from the junction of the stomatodeum with the respiratory chamber. 
We see then, so far, that the comparison of the vertebrate nervous system 
with the conjoined central nervous system and alimentary canal of the arthropod 
has led to a perfectly consistent explanation of almost all the peculiarities of the 
head region of Ammoccetes. We have solved the segmentation of the skull and the 
mysteries of the cranial nerves, for we have found that the cranial segmentation of 
the vertebrate can be reduced to the segmentation of the prosomatic and mesoso- 
matic regions of the Limulus, that the cranial skeleton arose from the modified 
internal chitinous skeleton of the Limulus, that the new mouth was formed by the 
forward growth of the metastoma, leading to the formation of an oral chamber, 
while the old mouth remained as the hypophysial tube, guarded by its olfactory 
and taste organs. 
Search as we may in the prosomatic and mesosomatic regions of scorpion-like 
animals, there are but few points left for elucidation; among these the most 
important are, 1, the fate of the coelomic cavities and coxal gland ; 2, the fate of 
the heart ; 3, the fate of the external chitinous covering. 
Comparison of the Head Cavities of the Vertebrate with the Prosomatic 
and Mesosomatic Celomic Spaces of Limulus. 
A recent paper by Kishinouye ' on the development of Limulus enables us to 
compare the coelomic cavities in the head region of a vertebrate with those of the 
prosomatic and mesosomatic segments of Limulus, and we see that the comparison is 
wonderfully close ; for whereas each mesosomatic segment possesses a coelomic cavity, 
just as each of the segments of the branchial chamber supplied by the vagus, glosso- 
pharyngeal, and facial nerves possesses a coelomic cavity, this is not the case with the 
prosomatic segments. In these latter the first coelomic cavity isa large preeoral one, 
common to the segment of the first appendage and all the segments in front of it; 
the segments belonging to the second, third, and fourth appendages have no coelomic 
cavities formed in them, the second ccelomic cavity belongs to the segment of the 
fifth appendage. Similarly in the vertebrate in the region corresponding to the 
prosoma there are only two head cavities recognised, viz. the Ist przoral head 
cavity of Balfour and V. Wijhe ; and 2nd or mandibular head cavity, associated 
especially with the Vth nerve. According to my view the motor part of the Vth 
nerve represents the locomotor prosomatic appendages of Limulus, and we see 
that already in Limulus the three foremost of these appendages do not form 
cceelomic cavities. 
In fact, the agreement in the formation and position of the ccelomic cavities in 
the head region of the vertebrate and in the prosomatic and mesosomatic regions 
of Limulus could not well be more exact; further, these cavities agree in this, that 
in neither case are they permanent; both in the vertebrate and in the arthropod 
they are supplanted by vascular spaces. 
1 Kishinouye, Jowrn. of Coll. of Sci. Tokio, vol. v. 1891. 
