1866.] 19 [Agassiz. 



the second, third and fourth, (beginning at the anterior extremity), 

 unite on the ventral side, while on the dorsal side the muscular bands 

 are nearly equidistant. (See fig. 1). The anterior opening for the 

 admission of water is by far the largest of the two openintrs of the 

 respiratory cavity ; it occupies the whole width of the body (fio-s. i- 

 2), while the posterior one, through which the water is expelled is 

 much narrower, and placed at a short distance from the posterior ex- 

 tremity, at the base of the truncating plane, on the ventral side, the 

 anterior opening (a.) being nearer the dorsal side. The lips which 

 close these openings are quite prominent, and can be thrown out 

 considerably beyond the general outline, either when drawing in 

 water, or forcing it out ; the lips of the anterior aperture open dor- 

 sally, those of the other end open in the opposite direction (6.) when 

 in action. (Fig. 3). 



The external and internal tunics are well defined ; hollowed out 

 from this internal tunic arise the circulating veins ; the larger ones 

 are especially apparent at the point where the muscular bands are im- 

 bedded in this tunic. The gill ((7.) runs nearly parallel with the ventral 

 surface, in the shape of a thick, hollow column, wider at the posterior 

 extremity ; it communicates dorsally and ventrally with the circulat- 

 ing system, and is strongly ribbed on the back and sides with bands of 

 cilia. At the anterior end of this gill is placed the languet (L), a long 

 grooved conical process slightly s-shaped, extending to the dorsal side 

 of the respiratory cavity, and hanging freely within it, attached by a 

 broad base to the ventral side of the cavity ; the base of attachment 

 is formed by a widening of the foot of the languet, where we find the 

 ciliated fossa (c./.) The large triangular area (figs. 1 and 2) within 

 which the languet is placed, extends from the anterior end of the gill, 

 and the vibratile cord (c. b.) which defines this vibratile cavity, as 

 the area is called, encircles completely the anterior extremity of the 

 respiratory system, and occupies about one quarter of the length of 

 the body. At the angle made by the vibratile band, forming the edge 

 of this cavity, where the cord takes a dorsal direction, is placed the 

 nervous ganglion (n. g.), immediately behind the base of the lano-uet ; 

 the ganglion is quite prominent, and sends numerous branches to the 

 walls of the body. A large vesicle attached to the ganglion contains 

 three irregularly shaped calcareous bodies, with deep black pio-ment 

 spots on the exterior side, making the nucleus a most prominent object 

 at the bottom of the ciliated cavity. The endostyle (e.) occupies 

 nearly two-thirds of the length of the body. The heart (h.) is very 

 prominent ; it is placed slightly to one side of the median line, above 

 the nucleus. 



Surrounding the nucleus (n.) is found a chain of diminutive Salp«, 

 (ch.) extending in a circle from the right side of the nucleus on the up- 



