THE CEPHALOCORDA 179 



is known as the endostyle. It is lined throughout by very 

 attenuated columnar ciliated cells, and on either side of 

 the middle line are two tracts of glandular cells. The cilia 

 at the bottom of the groove are especially long, and serve to 

 drive along the cord of mucus secreted by the glandular 

 cells'. Amphioxus, as has been explained, feeds upon minute 

 organisms contained in the currents of water passing in at 

 the mouth. On entering the pharynx, these organisms are 

 entangled in the strings of mucus driven forward by the 

 cilia of the endostyle, are passed by the ciliary action of the 

 peripharyngeal bands to the hyperbranchial groove, and are 

 carried down it to the intestine, while the water escapes 

 through the gill-slits into the atrial chamber, and thence to 

 the exterior by the atriopore. 



The notochord, as has already been seen, extends from 

 one end of the animal to the other, and anteriorly projects 

 well in front of the mouth. This anterior extension of the 

 notochord is so , characteristic a feature, that Amphioxus and 

 the genera nearly related to it are placed in a separate class, 

 Cephalocliorda. Throughout the region of the gut the 

 notochord lies close above the roof of the latter. It is 

 formed from a median groove in the dorsal wall of the 

 primitive gut of the embryo, the groove being afterwards 

 separated off as a cord of cells which undergo remarkable 

 histological changes, and give rise to a peculiar form of 

 supporting tissue known as notochordal tissue. In the adult 

 the notochord seems to be made up of a number of vertical 

 discs separated from one another by irregular vacuolar spaces. 

 The nuclei of the cells which formed the notochordal tissue 

 can be seen in transverse section to form dorsal and ventral 

 groups lying near the periphery of the cord. 



The notochord is surrounded by a stout connective tissue 

 sheath which is produced upwards to form an investment 

 to the nervous tube, and this investment is again prolonged 

 dorsally into a median band of fibrous connective tissue, which 

 in turn is continuous with the fibrous sheaths of the fin-rays. 

 Laterally the notochordal and neural sheaths and the median 

 fibrous band are continuous with the connective tissue septa 

 separating the muscle segments or myotomes. 



A transverse section shows that the upper part of the body, 

 right and left of the notochord and central nervous system, is 



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