THE LANGELET. 139 



The mouth is OTal, surrounded with a circle of ciliated 

 tentacles supported by semi-cartilaginous processes arising 

 from a circumoral ring. The mouth leads directly into a 

 large broad pharynx or " branchial sac" (Fig. 18.3, g), pro- 

 tected at the entrance by a number of minute ciliated lobes. 



The walls of this sac are perforated by long ciliated slits, 

 comparable with those of the branchial sacs of Ascidians 

 and of Balanoglossiis. The water which enters the mouth 

 passes out through these slits where it oxygenates the blood, 

 and enters the peribranchial cavity, thence passing out of 

 the body through the abdominal pore (Fig. 183, p). The 

 pharynx leads to the stomach (/), with which is connected 

 the liver or coecum. There is a pulsatile vessel or tubular 



Fig. 183. — o, vent; /, stomach; g, phaTTUx; «, nervous cord; j>, pore; r, noto- 

 cord; i, tentacles. From Llitken's Zoology. 



heart, beginning at the free end of the liver, and ex- 

 tending along the under side of the pharynx, sending 

 branches to the sac and the two anterior branches to the 

 dorsal aorta. "On the dorsal side of the pharynx the 

 blood is poured by the two anterior trunks, and by the 

 branchial veins which carry away the aerated blood from 

 the branchial bars, into a great longitudinal trunk or 

 dorsal aorta, by which it is distributed throughout the 

 body." (Huxley.) There are also vessels disti'ibuted to 

 the liver, and returning vessels, representing the portal 

 and hepatic veins. The blood-corpuscles are white and 

 nucleated. 



The vertebral column is represented by a notocord 

 which extends to the end of the head far in front of the 

 nervous cord; and also by a series of small semi- cartilagin- 

 ous bodies above the nervous system, and which are thought 

 to represent either neural spines or fin-rays. The nervous 

 cord lies over the notocord; it is not divided into a true 



