38 



AMPHIOXUS. 



iiotochord and spinal cord, connective tissue partitions or septa 

 arise, which, running outwards to the skin, divide the great 

 lateral muscles of the body into muscle-segments or myotomes 



(Fig. 11,K, and Fig. 12,x.) 

 The attachments of these 

 septa to the skin are indi- 

 cated by a series of > - 

 shaped markings, very 

 clearly seen on the sides 

 of the animal along its 

 whole length (Fig. 11). 



The only other skeletal 

 structures of importance 

 are a series of elastic chiti- 

 nous rods, supporting the 

 side walls of the pharynx ; 

 and an oval hoop, sur- 

 rounding the mouth. 



The great lateral mus- 

 cles, noticed above, are the 

 most important part of the 

 muscular system. They 

 form the side walls of the 

 body along its whole length 

 (c/. Figs. 12 and 13), 

 and are divided, as already 

 described, into muscle seg- 

 ments or myotomes by the 

 connective tissue septa. 

 The muscle fibres of each 

 myotome run longitudi- 

 nally, i.e. parallel to the 

 axis of the body, the fibres 

 taking origin from the 

 connective tissue septa. 

 The myotomes have been 

 found to be sixty-one on 

 each side of the body in a 

 considerable number of specimens, and it seems probable 

 that this number is constant. The myotomes of the two sides 



