24 PHYLUM CEPHALOOHORDA, 



har. The primary gill-slits of opposite sides alternate with one 

 another, as do the myotomes. In the adult they are more 

 numerous than the myotomes, though when they first make their 

 appearance, they correspond with them. The anterior primary 

 slit on each side is not divided by a tongue bar. 



On account of the obliquity of their direction a great many, 

 both of primary and tongue bars, are cut in transverse section 

 (Fig. 10). The primary bars differ from the tongue bars in 

 structure (Fig. 11, A, B). In both there is an axial rod of a 

 clear chitin-like substance (insoluble in potash), placed nearer 

 the atrial than the pharyngeal side of the bar. In the primary 

 bar this rod is double and without a cavity, while in the tongue 

 bar it is single and has a cavity. These rods are continued inwards 

 towards the pharynx as a thin membrane called the septal 

 membrane (Fig. 11). The character of epithelium covering 

 the bars may be gathered by an inspection of Fig. 11. On the 

 outer side the ectoderm of the atrium is found. It is said to be 

 non-ciliated and to be separated from the more extensive pharyn- 

 geal endoderm by some pigmented cells. The cilia of the endo- 

 derm vary in character on different portions of the bar, recalling 

 the condition found in the Lamellibranch gill. The primary 

 bar contains between the ectoderm and the skeletal rod a chamber 

 which is a porti.on of the coelom, being continued dorsally into 

 the dorso-pharyngeal coelom, and ventrally into the endostylar 

 coelom. In addition three blood-vessels can be seen in the 

 primary bar in the jjositions and with the names indicated in the 

 figure. In the tongue-bar the visceral (10) and somatic (7) 

 vessels are present, and in addition there is a space in the 

 skeletal rod (3). This is interpreted by Lankester and Benham 

 as coelomic, by others as vascular. According to Benham this 

 space contains a blood-vessel (omitted in the figure) corresponding 

 to the external blood-vessel of the primary bar (4). Successive 

 primary bars are connected by transverse bars {synapticula), 

 which thus pass across the primary slits, internally to the tongue 

 bars, with which, however, they are connected (Fig. 12, s). The 

 primary bars may thus be distinguished from the tongue bars 

 in transverse section ; and they may be distinguished by in- 

 spection of the pharynx as a whole for the skeletal rod of the 

 primary bars bifurcates ventrally on reaching the level of the en- 

 dostyle, whereas that of the tongue bars does not bifurcate. Dor- 



