STRUCTURE OF AMPHIOXUS. 367 



It is likely that the most important sensory structures are 

 the sensitive cells scattered in the epidermis. 



Alimentary System. — The mouth is usually exposed when 

 the animal is lying in the sand. It is fringed by about 

 thirty ciliated tentacles or cirri, which are supported by fila- 

 ments from the segments of the mouth bars. 



Within the mouth lies a membranous flap or velum, 

 separating the cavity of the mouth from that of the pharynx, 

 but perforated by an aperture, through which mouth and 

 pharynx communicate. The aperture of the velum seems 

 to represent the mouth of the larva. 



The pharynx, like that of Tunicates, and indeed of Fishes 

 also, is modified for respiration. Its walls are perforated by 

 numerous gill-slits on each side, and between these lie sup- 

 porting bars or arches, alternately split and unsplit. The 

 internal surface of the pharynx, like that of the mouth, is 

 richly ciliated. 



Along the mid-dorsal and mid-ventral lines there are 

 grooves, hyper- and hypo-branchial respectively. The latter 

 is comparable to the endostyle of Ascidians. 



The intestinal region of the gut is straight and simple, 

 except that near its commencement, a pouch-like " liver " 

 arises and extends forward on the right side of the pharynx. 

 The anus lies rather to the right side. 



Body-Cavity. — This arises as a paired and subsequently 

 transversely divided pouch from the gut or archenteron of 

 the embryo. Its extent is not very easily appreciated in the 

 adult, especially in the region of the atrial cavity. 



Posteriorly the body-cavity is the wide space between the 

 intestine and the body walls. In the anterior region it is 

 reduced to small canals, which lie above and by the side of 

 the pharynx, and within the metapleural folds or lateral fins. 

 Besides the main canals, there are others which replace the 

 lymphatic system of higher animals. The cavities contain 

 coagulable fluid, and are in some regions continuous with 

 the blood-vessels. 



Respiratory System. — The water which enters the mouth 

 and is swept down the pharynx passes out by the gill-slits. 

 In the embryo it regains the exterior directly ; in the adult 

 it is received into an atrial chamber, which opens by the 

 single atriopore. This atrial chamber is formed by the 



