462 SUB-PHVLUM CEPHALOCHORDA 
abundant on the buccal cirri. The epidermis lies upon a 
thin layer of clear cutis. 
Beneath this there is a layer of fine tubes, which unite in a longitudinal 
canal running along each metapleural fold. These metapleural canals 
are said by some to arise in development by a splitting of an originally 
solid mass (schizoccelic); but it seems more probable that they are 
morphologically portions of the true coelom—ventro-lateral extensions 
of the ‘‘ collar-ccelom ” (enteroccelic). 
Skeleton.—This is slightly developed, for there is not 
only no bone, but the material is not even definitely 
cartilaginous. It may be called “ chordoid ” tissue. 
(a) The notochord runs from tip to tip. It consists of 
vacuolated cells, and the supporting power is probably due 
to their turgidity, as in many vegetable structures. Its 
anterior extension beyond the end ofthe nerve-cord is 
particularly characteristic. 
(2) The pharynx is supported by chitinoid bars, which 
border the numerous gill-slits. There is also a series of 
paired plates underlying the mid-ventral groove. 
(c) The margin of the pre-oral hood contains a supporting 
ring, segmented into about two dozen pieces, each of which 
sends a process into the adjacent cirrus. 
Connective tissue—The sheath which envelops the 
notochord and is continued round the nerve-cord, the septa 
of connective tissue (myocommas) which divide the muscle 
segments, and the numerous “fin rays” which support the 
dorsal and ventral fins, may be noticed here. 
Muscular system.—The sixty-two muscle segments, myo- 
tomes, or myomeres, are dovetailed into one another like a 
succession of V-shaped plates, and are particularly strong 
dorsally. These produce the side-to-side wriggling move- 
ments by which the animal swims. On the ventral surface, 
between the mouth and the atriopore, there is a transverse 
set of fibres, which help to drive out the water from the atrial 
cavity. Other muscles occur in the region of the mouth, 
and elsewhere. Most, if not all, of the fibres are striated. 
Nervous system.—The dorsal nerve-cord is shorter than 
the notochord, and has no definite brain. In the anterior 
region, however, there is some differentiation in minute 
structure, and the central canal widens out to form the so- 
