vu VERTEBRAL COLUMN 195 
(haemal arches) enclosing a haemal canal (Fig. 112, B). The apex 
of each arch is prolonged into a median process or haemal spine. 
Although not recognisable in the Dog-Fish, paired inter-ventral 
cartilages, corresponding with the inter-dorsals above, are present 
in some Elasmobranchs and alternate with the basi-ventrals. In the 
caudal region of others (e.g. Skates and Rays) ventral counterparts 
of the supra-interdorsals are present, and are termed infra-ventral 
cartilages. Much in the same way that their dorsal equivalents 
enclose a neural canal, so the ventral arcualia partially surround the 
viscera-containing coelom in the trunk; and in the tail, but more 
completely, the 
vestigial coelom 
of that region or 
the haemal canal. 
The different 
vertebral ccmpon- 
ents are by no 
means of equal 
morphological 
value. The basi- 
dorsals and basi- 
Fie. 112.—A, transverse section of a precaudal vertebra; B, 
ventrals, and the similar section of a caudal vertebra. h.a, Haemal arch 
inter-dorsals and (basi-ventrals) ; h.c, haemal canal ; h.s, haemal spine ; x.¢, 
: neural canal. Other reference letters as in Fig. 111. 
inter-ventrals, are Bs 
the primary elements and the most important. The supra-dorsals 
are merely cartilages segmented off from the basi-dorsals and inter- 
dorsals, while the ribs and the infra-ventrals are similarly derived 
from the basi-ventrals and inter-ventrals respectively. As to the 
vertebral elements which collectively form a vertebra in the Dog- 
Fish, it would seem from evidence afforded by the neuromeres,’ and 
more especially by the facts of development, that each complete 
skeletal segment or vertebra consists of a pair of basi-dorsals with 
the preceding pair of inter-dorsals, and of a pair of basi-ventrals 
with the next succeeding pair of inter-ventrals. It must be 
emphasised, however, that, considered as a joint or segment in a 
flexible back-bone, a vertebra is a physiological unit, the morpho- 
logical value of which may differ widely in different Fishes. 
Hence, in other Fishes, the grouping of vertebral components to 
1 Neuromeres are body-segments defined and limited by the exits of the suc- 
cessive pairs of spinal nerves from the neural canal. 
