442 SUB-PHYLUM CEPHALOCHORDA. 



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-coelom " (enteroccelic). 



Skeleton.- -This is slightly developed, for there is not only 

 no bone, but the material is not even definitely cartilaginous. 



(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. 



(/>) 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. 



(cT) 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 also 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. Nearly all 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- 

 called cerebral vesicle, which in the larva communicates 

 with the anterior by a pore (the neuropore). From the 

 nerve-cord there arise two sets of nerves, dorsal and ventral. 

 Of these the two anterior pairs of dorsal nerves are called 

 cranial, and do not correspond to the myotomes. Behind 

 these a pair of dorsal nerves arise at each myotome, but, as 



