22 PHYLUM CEPHALOCHORDA. 



walls are perforated throughout their whole dorso-ventral 

 extent (except for a short distance behind the velum) by a 

 number of vertically directed slits, which have a slight inclination 

 ventralwards and backwards (more marked in the preserved 

 than in the livmg animal), and which open into the atrial cavity. 

 The dorsal and ventral parts of the pharyngeal wall are not 

 perforated and constitute the hyperpharyngeal groove and the 

 endostyle (hypopharyngeal groove) respectively. These termi- 

 nate independently of one another posteriorly, but anteriorly 

 they are connected by the peripharyngeal ciliated bands which 

 arch round the pharynx immediately in front of the gill -slits. 

 In front of the peripharyngeal bands there is a small portion 

 of the pharnyx adjacent to the velum without gill-slits. 



The pharynx is lined by a ciliated epithelium, which is con- 

 tinuous through the slits with the ectodermal epithelium lining 

 the atrial cavity. Along the endostyle there are four bands of 

 specially glandular cells, which, like the remaining pharyngeal 

 cells, bear cilia, and secrete the mucus, which passing forwards 

 along the endostyle is driven upwards by the peripharyngeal 

 bands into the front end of the hyperpharyngeal groove. Along 

 this it is carried by ciliary action into the stomach. The food, 

 consisting of small floating organic bodies brought into the 

 pharynx by the ciliary currents, is entangled in this mucus and 

 so separated from the water which passes through the gill-slits 

 into the atrial cavity and out by the atrial pore. 



New gill-slits continue to be formed long after the development 

 has ceased, during the growth of the animal. They are conse- 

 quently more numerous in large than in small specimens. In 

 large specimens there may be as many as 180 secondary (see 

 below) or 90 primary gill-slits on each side. 



The new slits are formed at the hind end of the pharynx close 

 to the junction with the stomach, as small circular perforations 

 (see Fig. 24 in the account of the development). These soon 

 become partly divided into two by the growth ventralwards of a 

 process from the dorsal wall of the aperture (Fig. 25). This 

 downward projection, from its resemblance to the tongue of a 

 Jew's harp, is called the tongue bar. It eventually fuses with the 

 ventral wall of the slit, so that the primary slit becomes completely 

 divided into two secondary slits separated by the tongue bar. 

 ' In correspondence with this we may call the parts of the pharyn- 



