I42 FISHES CHAP. 
the anterior end of the brain. In most Craniates, however, the 
notochord becomes more or less completely replaced in the adult 
by the development round it of a series of vertebrae, forming the 
backbone or vertebral column. ‘Two pairs of limbs, and cartila- 
ginous or bony limb-girdles for their support, are very generally 
present. 
The segmentation, or serial repetition of certain organs of the 
body, which is so marked a feature in the Cephalochordata, is 
also characteristic of the Craniata. Examples of this may be 
seen in the division of the lateral longitudinal muscles of the 
body wall into muscle-segments or myotomes by a series of 
transverse fibrous septa; in the formation of the vertebral 
column by a series of successive joints or vertebrae; in a similar 
serial repetition of the cranial and spinal nerves, the gill-clefts 
and branchial arches, certain blood-vessels, and the renal tubules. 
There is sometimes, however, no precise regional or numerical 
correspondence between the different organs which are successively 
repeated in this way, and hence it is probable that, in at least 
some of the organs of the Craniate body, the segmentation has 
been independently evolved in each case. 
The pharynx is relatively much shorter than in other 
Chordata. The gill-clefts are few in number, whether, as 
in the lower Craniata, they are retained as the functional 
breathing organs, or are present, as vestiges only, in the embryos 
of the higher members of the group. In no instance are they 
subdivided by the growth of “tongue-bars” or “synapticula,” 
nor do they open externally into an atrial or peribranchial 
cavity. The liver is a massive compound tubular gland, never, 
in the adult at all events, a simple caecal sac; and usually there 
is a pancreas and a spleen. 
A spacious epithelium-lined body cavity or coelom, which, as 
regards its origin, may be regarded as a “syncoelom,” ! surrounds 
the alimentary canal and separates it from the body wall. From 
the epithelial walls of the coelom are derived the gonads (ovaries 
and testes), which in the adult are limited to a single pair; while 
paired and often seementally-arranged lateral tubular outgrowths 
from it (renal tubuli) acquire a glandular character and form 
the basis of the excretory or kidney system. A special portion 
1 A coelom formed by the union of one or more pairs of primitively distinct coelomic 
cavities. 
