W. K. BROOKS ON THE GENUS SALPA. 



31 



imbedded in visceral follicle cells. Plate XXII shows a series of trans- 

 verse sections at the same stage, but as these are not perfectly vertical 

 but in planes which make an acute angle in front with the horizontal 

 axis, they are less easily intelligible than the cut. Fig. 2 passes through 

 the pharynx, c ; Fig. 3, through the two gill-tubes, g", and Fig. 4, through 

 the cloaca, g'", and the two spiracular tubes, g", g". The details of the 

 formation of the pharynx and gill-tubes and gill, o, are shown in Plate 

 XLII, Figs. 6, 7 and 8. The cavity of the pharynx, Fig. 8, c, and Plate 

 XIV, Figs. 8 and 9, c, and Plate XXII, Fig. 2, c, is hollowed out in the 

 mass of visceral follicle cells, below the cloaca, by the degeneration of 

 the follicle cells. These become amoeboid and are set free in the cavity, 

 where they persist for some time. They are easily recognizable by the 

 transparency of their nuclei, and by the fact that these are usually in pairs. 



3 Plate 14 



CUT D. 



A reconstruction in a vertical transverse plane from the horizontal 

 section shown in Plate XIV. 



The communication between the perithoracic tube g" and the 

 pharynx is formed in essentially the same way. The somatic follicle 

 cells, 7, of the perithoracic tube, Fig. 6, Plate XLII, and Fig. 8, and 

 the visceral follicle cells between it and the pharynx, Fig. 8, 8, become 

 amoeboid, and wander out into the cavity which is thus formed. The 

 rod-like mass of cells, Fig. 8, o, which is left between the cloaca g, above, 

 and the pharynx c, below, and the gill-slits g", on the sides, is the so-called 

 gill. It consists of a mass of visceral follicle cells, 8, which contains 



