THE CEPHALOCORDA 201 



elastic rod running through the length of the body dorsal 

 to the gut. Mechanically this rod, the notochord, serves 

 as a support to the body, and as an attachment to the 

 muscles. Accordingly we find a great development of dorsal 

 musculature, accompanied by the reduction and obliteration 

 of the ccelom in this region. A similar elastic rod is found 

 in the embryos of all higher vertebrates. In Amphioxus the 

 notochord is simply invested by a connective tissue sheath. 

 In higher vertebrates we have a progressive series in which 

 the notochord is (i) surrounded by continuous cartilage, 



(2) the cartilage is divided into segments or vertebral centra, 



(3) the centra are thickened with consequent reduction of 

 the notochord, (4) the cartilage is replaced by bone and the 

 notochord disappears, except for a few traces. The significance 

 of these facts cannot be overrated. 



The second characteristic vertebrate feature in Amphioxus 

 is the dorsal position and tubular character of the central 

 nervous system. This tubular character is referable to the 

 mode of its development, and we find that this mode is 

 similar throughout the vertebrate phylum. 



A third and very important vertebrate feature is the 

 presence of gill-slits, structures which are always present in 

 the embryos of vertebrates, but are obliterated in the adults 

 of air-breathing forms. Amphioxus is remarkable for the 

 vast number of its gill-slits ; this is most probably a secondary 

 feature connected with its habits and mode of feeding. 

 Correlated with the presence of gill-slits, serving as respiratory 

 organs, is the vertebrate type of circulation, a contractile 

 ventral blood-vessel connected by lateral vessels passing up 

 between the gill-slits with a dorsal blood-vessel in which 

 the blood flows backwards. In this connection we may 

 also notice the presence of a hepatic portal system. We 

 may further notice as features that occur in, but are not 

 peculiar to vertebrates, the segmentation of the body expressed 

 by the myotomes and the presence of metamerically repeated 

 excretory tubules. It is, however, a peculiar vertebrate feature 

 that the primitive segmentation is retained only in the dorsal 

 musculature or myotomes, but is lost in the ventral region 

 through fusion of the splanchnoccelic subdivisions of the 

 primitive somites. 



But Amphioxus, while exhibiting so clearly the fundamental 



