142 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," l 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 segmentally-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. 



