MUSCLES SKELETON 



119 



Classification, p. 137, where a list of the species with the number 

 of myotomes in each is given.) 



There are also transverse muscles (Fig. 72, mt) extending 

 across the ventral surface in the region of the body enclosed by 

 the metapleural folds, and serving to compress the atrial cavity, 

 and so aid in the expulsion of its contents. 



Outside the muscular layer of the body-wall the thin in- 

 tegument is formed of a dermal layer of soft connective tissue, 

 covered by the epidermis, a single layer of columnar cells, many 

 of which, especially on the oral cirri, have sensory bristles. 



Skeleton. — The endoskeleton consists of the notochord and 

 some tracts of modified connective tissue which support various 

 parts of the body. 



The notochord of this animal is noteworthy amongst Chordata 

 for extending practically the entire length of the body, including 

 the head, from snout to tip of tail (Fig. 71). It lies in the 

 median plane, but nearer the dorsal than the ventral surface 

 (Fig. 72), and has the myotonies at its sides, the nervous system 

 above and the alimentary canal below. It is elliptical in 

 section, and tapers to the two ends. The nuclei of the original 

 notochordal cells are displaced to the dorsal and ventral edges, 

 and the greater parts of the 



cells, in the adult, are occu- 

 pied by large vacuoles filled 

 with a fluid secretion, so as 

 to form by their distended 

 condition a stiff elastic struc- 

 ture. This state of the cells, 

 and the appearance it gives 

 rise to (Fig. 73), seen best 



Fio. 73. 



Median sagittal section of notochord 



of an Amphioxus of 32 mm. 



m young specimens, is very 



characteristic of notochordal 



tissue. Around the notochord lies a sheath of connective tissue 



which is continuous with the similar sheath around the nervous 



system and with the septa between the myotomes. 



In addition to these skeletal layers of connective tissue there 

 is a cartilage-like tract in the oral hood. This is jointed, or 

 made up of separate rod-like pieces, one at the base of each 

 cirrus, into which it sends a prolongation (Fig. 71, sk). The 

 dorsal and ventral fins are supported by single and double rows 



