I2O CEPHALOCHORDATA 



respectively of what have been called " fin-rays." They are short 

 rods of gelatinous connective tissue, each enclosed in a lymph 

 space. Finally, the bars constituting the walls of the pharynx 

 between the gill-slits contain slender skeletal rods which run 

 obliquely dorso-ventrally, and are of a stiff, gelatinous nature (see 

 Fig. 75, p. 122). This skeletal connective tissue consists in all 

 cases of a fibrous deposit or matrix produced by the layer of 

 epithelium (ectodermal, endodermal, or mesodermal) which adjoins 

 the tissue. 



Alimentary Canal. This has, as its most noteworthy feature, 

 the Chordate characteristic that the pharynx gives rise to the 

 respiratory organ (see Figs. 71 and 74, A) ; and in size and pro- 

 minence, both in side view and in sections, the modified pharynx 

 of Amphioxus is fairly comparable with the branchial sac 

 (pharynx) of many Tunicata (see Fig. 23, p. 51), and might be 

 called by the same name. 



The small primitive mouth, at the bottom of the cavity 

 bounded by the oral hood (stomodaeum), has a membranous 

 border, the velum (Fig. 71, r/), the edges of which are prolonged 

 into a circle of 10 or 12 (up to 16 in some species) simple oral 

 tentacles turned inwards towards the pharynx (compare tentacles 

 of Ascidians, p. 45). 



The pharynx, by far the largest part of the alimentary canal, 

 and extending nearly half-way along the body, is more important 

 as a respiratory than as a nutritive organ. Its walls over nearly 

 the whole extent are perforated by a large, and indefinite, 

 number (100 -or more on each side) of gill-slits which run on 

 the whole dorso-ventrally, but in the contracted condition seen 

 in preserved specimens have their lower ends directed obliquely 

 backwards, so that a vertical transverse section may cut through 

 a number of such slits and the intervening branchial bars (Fig. 

 74, A, &&). These bars, and therefore the slits between them, 

 are of two orders, primary and secondary, the latter being devel- 

 oped later in larval life as downgrowths or " tongue-bars," one 

 from the top of each primary gill-slit, so as to divide it into 

 two secondaries. The primary and the secondary (or tongue-) 

 bars can be distinguished from one another by their structure in 

 the adult animal (Fig. 75, A and B). 



It must be remembered that these branchial bars, or septa 

 between the gill-slits, are not merely portions of the wall of the 



