16 PHYLUM CEPHALOCHORDA. 



cartilaginous matrix. The tissue of the axial rods of the pharyngeal bars is 

 sometimes described as skeletal. It consists of a clear chitin-like substance 

 devoid of cellular structures except in the ventral bifurcated parts of the 

 primary bars, which contain branched cells. Excluding the nuclei of the 

 notochord, which is an endodermal structure, this is the only instance 

 of a mesodermal tissue containing mesenchymatous elements. 



The nerve-cord is surrounded by a tough sheath, which is 

 continuous with the sheath of the notochord. 



The fin-rays are found in the greater part of the dorsal 

 fin in a single series, and in the anal fin in a double series (Figs. 

 4 and 5). They are absent from the cephalic fin, and from the 

 anterior and posterior portions of the dorsal fin, and from the 

 ventral parfc of the caudal fin. They consist of small cubical 

 pieces of a tough fibrous connective tissue, which in the dorsal 

 fin are continuous with the fibrous investment of the nerve- 

 cord. They are more numerous than the metameres, there 

 being four or five to each muscle segment. 



The fins contain a longitudinal canal lined by epithelium and divided 

 by septa into compartments. In the region of the fin-rays each of these 

 compartments contains a fin-ray which pushes in its ventral wall and 

 projects into it. The nature of these fin spaces is not known, but it is 

 stated by Hatschek and others that they are, in the dorsal fin at least, 

 coelomic in origin, being derived from the coelom of the muscle-plate (see 

 below). 



The muscular system consists of striated and unstriated 

 muscular tissue. The striated muscles are composed of fibrillated 

 rhombic plates, and are devoid of sarcolemma. They con- 

 stitute the lateral muscles, the transverse muscles, the muscles 

 of the lips and cirri, and the sphincter muscle of the velum and 

 anus. 



The great lateral muscle of the body is divided up into a 

 number of successive segments, the myotomes (myomeres) 

 by septa of connective tissue. These septa have a peculiar 

 V-shaped course, and their insertion into the skin causes the 

 V-shaped external markings already referred to (Fig. 4). The 

 myotomes of opposite sides alternate with one another, i.e. 

 the intermuscular septa of one side are opposite the middle 

 of the myotomes of the other side. The full number of 

 myotcmes is laid down in the embryo. In Amphioxus lanceolatus 

 there are about sixty-two on each side. The plate-like n, res 

 of which these muscles are composed extend the whole length 



