34 ANATOMY OF AMPHIOXUS. 



bar of the pharynx. Wedged in between the ccelomic and 

 atrial epithelia of the primary bar is a small blood-vessel, 

 v. Internal to the coelomic space lies the skeletal rod, 

 which in section has the shape of a triangle, at whose apex 

 there is another blood-vessel, v". The sides and inner 

 edge of the bar are composed of the ciliated pharyngeal 

 epithelium. The cells of the latter are always arranged in 

 a single layer, but at the sides of the gill-bars they are 

 very long and thin, and the nuclei are crowded together at 

 different layers so as to give the idea of a many-layered 

 epithelium (Fig. 15 C). The cells of one side of the bar 

 are in juxtaposition with those of the opposite side, except 

 at a point near the internal edge of the bar, where a space 

 occurs. In this space there is a third blood-vessel, v' n '. 6 



In the secondary bar, there is no vessel corresponding 

 to the one marked v in the primary bar, and the vessel 

 that corresponds with v" is entirely enclosed within the 

 skeletal rod. 



The dorsal wall of the pharynx is closely appressed 

 against the sheath of the notochord, and separates the two 

 dorsal aortae from one another. It has here the form of a 

 groove running parallel with and opposite to the endostyle. 

 It is known as the hyperbranchial groove. (Cf. Fig. 2.) 

 Two special tracts of ciliated epithelium form the sides of 

 it, and pass downwards in front to join the anterior extrem- 

 ity of the endostyle on each side. In front, where these 

 tracts bend downwards with a crescentic curve, they are 

 known as the peripharyngeal bands. (See Fig. n, ///./;.) 



Musculature. 



The musculature of Amphioxus is composed almost 

 entirely of striated muscle-fibres. Involuntary or smooth 

 muscle-fibres are remarkable for their extreme tenuity, and 



