X AMPHIOXUS 393 



Such movement as takes place is carried out in the primitive verte- 

 brate fashion by lateral flexure of the body. In correlation with this 

 the longitudinal muscles are segmented into myotomes — of a simple 

 V-shape, and the edge of the body is extended slightly into a ridge 

 although it is only in the (protocercal) tail region that this is so thin 

 and flat as to be worthy of being called a fin. There is no trace of 

 paired fins but throughout the anterior portion of the body the surface 

 bulges out on each side ventrally to form the metapleure (Fig. 172, m) 

 enclosing in its interior a large lymph space. 



The buccal cavity or stomodaeum is a gaping funnel-like opening on 

 the ventral side anteriorly. It is fringed by a number of slender tentacles 

 or cirri and its lining, especially posteriorly, carries powerful flagella 

 which serve to drive a current of water with floating food-particles back 

 into the pharynx. The boundary between stomodaeum and pharynx is 

 formed by a vertical partition — the velum (Fig. 172, v) — perforated by 

 a circular opening from whose lips there project back into the pharynx 

 a circle of twelve velar tentacles. 



The pharynx is exceedingly large in Amphioxus and presents striking 

 peculiarities related in all probability to the nature of the food — micro- 

 scopic particles floating in the sea-water. The gill-clefts are exceedingly 

 numerous, the series apparently being added to throughout life by the 

 addition of new clefts at the posterior end. Further each cleft, at first 

 rounded in form, becomes split into two by a tongue-like downgrowth 

 from its dorsal wall. The clefts also become greatly elongated in a 

 dorso-ventral direction, forming exceedingly narrow slits, through which 

 water is drawn by powerful cilia on their edges, but which do not permit 

 food-particles to pass. Along the mid- ventral line the pharyngeal floor 

 forms a longitudinal groove or gutter, the lining epithelium being partly 

 ciliated, partly glandular, secreting sticky mucus which passes into the 

 cavity of the pharynx and serves for the entanglement of food-particles. 

 This glandular organ — the endostyle — has, as we shall see later, had an 

 interesting fate in the evolution of the more typical vertebrates. The 

 shreds of sticky secretion, laden with food-particles, are carried dorsal- 

 wards by ciliary action until they reach a longitudinal groove lying along 

 the roof of the pharynx. The cells lining this are also ciliated and the 

 movement of their cilia produces a current by which the mucus and 

 food-particles are slowly carried tailwards into the intestine. 



The intestine is a simple straight tube, somewhat dilated in front to 

 form the " stomach," which passes back and opens by the anus — 

 situated not in the mesial plane as is most usual in vertebrates but on 

 the left side (Fig. 172, an). Close to its front end the floor of the 



