THE GILL-SLITS AND THE ENDOSTYLE. 79 



2. The Endostyle. 



The eiidostyle appears at the commencement of the larval 

 period, or towards the close of the embryonic period, as a band 

 of columnar ciliated cells on the right side of the anterior end of 

 the pharynx, immediately in front of the club-shaped gland, and 

 in close contact with this. Its condition at an early stage of 

 the larval period is shown in Fig. 36, ES, where it is seen as a 

 broad > -shaped band, formed by modification of the hypoblast 

 cells of the right side of the pharynx, opposite the anterior part 

 of the mouth opening. The apex of the > is directed back- 

 wards ; the upper arm is much shorter than the lower ; and the 

 whole band is divided down its centre by a groove. 



In the later stages (Figs. 37 and 38, ES), the endostyle ex- 

 tends backwards, its apex passing behind the duct of the club- 

 shaped gland and making its way between the primary and 

 secondary series of gill-slits. As the critical stage is approached, 

 and the primary gill-slits shift across to the left side, the endostyle 

 (Fig. 39) moves to its permanent position on the mid-ventral 

 wall of the pharynx. At the same time it continues to extend 

 backwards, and at the critical stage has reached to about the 

 level of the fifth gill-slits. During the shifting of its position 

 the two limbs of the >, which were originally upper and lower, 

 become right and left respectively ; and as it extends back- 

 wards along the floor of the pharynx the two limbs become 

 closely applied, and fused together. From the anterior ends of 

 the limbs, a pair of ciliated ridges of epithelial cells extend up 

 the sides of the pharynx, and grow backwards along its dorsal 

 surface to form the epibranchial band of the adult. 



3. The Club-shaped Gland. 



The early stages in the formation of the club-shaped gland 

 have been already described, p. 70. The gland reaches its 

 maximum development about the commencement of the larval 

 period (Fig. 36), when it consists of a dilated sac, GL, lying on 

 the right side of the pharynx, and continuous with a narrow 

 tubular duct, which passes round the ventral surface of the 

 pharynx and opens to the exterior on the left side, close to the 

 anterior border of the mouth, GD. 



The dilated part of the gland soon becomes narrower, and 

 tubular, but according to Willey acquires an opening into the 



