954 REPORT—1896, 
Fic. 6.—Ammoceetes, Nerves of Visceral Segments and Cartilages. 
PROSOMA 
a! 
‘ 
ZZ) / 
NY 
MESOSOMA ¥ 
SUBCHORDAL BRANCHIAL 
CARTILAGINOUS CARTILAGES 
LIGAMENTS 
In all three Figures v,—v,=Prosomatice appendages and nerves ; vii=1st mesosoniatic ap- 
pendage or opercular appendage and nerves; ix, x,... = remaining mesosomatic 
appendages and nerves; M = Chilaria in Limulus, metastoma in Eurypterus. 
nature of the branchial chamber is seen in fig. 3, which demonstrates clearly 
that the [Xth and Xth nerves supply a series of separate gill-bearing struc- 
tures or appendages, which hang freely into a common respiratory chamber; 
each one of these appendages is moved by its own separate group of branchial 
muscles, and possesses an external branchial bar of cartilage, which, by its 
union with its fellows, contributes to form the extra-branchial basket-work so 
characteristic of this primitive respiratory chamber. The segmental branchial unit 
is clearly in this case, as Rathke originally pointed out, each one of these suspended 
gills, or rather gill-bearing appendages; it is absolutely unnatural, as Nestler! 
attempts to do, to take a portion of the space between two consecutive gills and 
call that a gill-pouch. It is, to my mind, one of the most extraordinary and con- 
fusing conceptions of the current morphology to describe an animal in terms of 
the spaces between organs, rather than in terms of the organs by which those 
spaces are formed. We might as well speak of a net as a number of holes tied 
together with string. Another most striking advantage is obtained by considering 
the segmental unit to be represented by each of these separate branchial append- 
ages—viz. that we can continue the series in the most natural manner (as seen in 
fig. 3) in front of the limits of the IXth and Xth nerves, and so find a series 
of appendages in the oral chamber serially homologous with the branchial append- 
ages. The uppermost of the respiratory appendages is the hyo-brancbial, supplied 
1 Nestler, Archiv f. Naturgeschich. 56, vol. i. 
