^02 THE LANCELET. PHYLUM CHORDATA 



one another so that several are cut in a transverse section. Within 

 the body-wall lies a perivisceral coelom, not divided by septa, but 

 greatly complicated by the presence of the gill slits, which reduce 

 it in the region of the pharynx to a number of canals in the 

 primary gill bars (p. 303), a ventral canal called the subendostylar 

 ccelom, and on each side a longitudinal dorsal coelom, all of 

 these being in communication with one another. There are 

 numerous ccelomic cavities, including those of the gonads. As 

 in vertebrates, the dorsal body-wall is much thicker than the 

 ventral. In it there hes a longitudinal, hollow central nervous 

 system, comparable to that of vertebrates, but at the front 

 end not enlarged into a brain, though the cavity, which behind 

 is narrow like the central canal of the vertebrate spinal cord, is 

 in front wide like that of the vertebrate brain. Below the central 

 nervous system, along the whole length of the body, lies an elastic 

 rod, the notochord, derived from the roof of the gut in the course 

 of development and bound to the nerve cord by a connective 

 tissue sheath which surrounds them both. It consists of highly 

 vacuolated cells and extends in front beyond the nerve cord to 

 the tip of the body. It assists in swimming in two ways. The 

 muscles of the two sides contract in a rhythmical and alternate 

 manner, and the notochord, being incompressible, converts such 

 unilateral contraction into a relatively sharp bending of the 

 whole body. When the contraction of the muscles of one side is 

 over, the elasticity of the notochord helps to produce a rapid 

 flick back. There is no skeleton of bone or cartilage, but stiff rods 

 of organic material support the gill bars and cirri, and the dorsal 

 and ventral fins. 



ALIMENTARY SYSTEM 



The mouth is surrounded by an oral hood (Fig. 227), the edge 

 of which is produced into a number of ciliated tentacles, or oral 

 cirri. Inside the hood is a buccal cavity or vestibule, which is 

 nearly closed posteriorly by a partition called the velum. The 

 opening in the centre of this leads into the pharynx, and is sur- 

 rounded by about a dozen backwardly-projecting velar tentacles. 

 Running round the buccal cavity just in front of the velum is 

 a lobed structure called the wheel organ, which is covered with 

 long cilia. Just in front of this there is a depression in the roof 

 of the cavity known as Hatschek's pit. The hood can be contracted 



