STRUCTURE OF AMPHIOXUS 517 



behind the mouth to the atriopore, and into which the gill- 

 sUts of the pharynx open in the adult. 



Skin. — The epidermis consists of a single layer of cylin- 

 drical cells. Some of them project slightly from the surface, 

 and are connected at the base with nerve-fibres. These are 

 sensory cells, and may be compared to the cells of the 

 lateral fine in fishes and tadpoles. Here, however, they 

 are scattered over the surface of the body, though especially 

 abundant on the buccal cirri. The epidermis Ues upon a 

 thin layer of clear cutis. 



Beneath this there is a layer of fine tubes, which unite in a longi- 

 tudinal 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 (schizocoelic) ; but by others to be ventrolateral 

 extensions of the " collar-coelora " (enterocoehc). 



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, 

 myotomes, 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 wriggUng 

 movements 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 



