462 SUB-PHYLUM 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 of the nerve-cord is 

 particularly characteristic. 



(b) 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- 



