38 



AMPIIIOXUS. 



notochord and spinal cord, connective tissue partitions or septa 

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

 lateral muscles of the bodv into muscle-segments or myotomes 



(Fig.ll,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 

 (cf. Figs. 12 and 13), 

 and are divided, as already 

 described, into muscle seg- 

 ments or myotomes by the 

 connective tissue septa. 

 pq . g The muscle fibres of each 

 x! 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 



