Iv MUSCLES—SKELETON I19Q 
Classification, p. 157, 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 myotomes 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 al IN 
condition a stiff elastic struc- 7 I\ | ¥ Ke \ \ 
ture. This state of the cells, ~ 
and the appearance it gives 
rise to (Fig. 73), seen best 
in 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 
Fia. 73.—Median sagittal section of notochord 
of an Amphioxus of 32 mm. 
