206 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



cloacal aperture can be described as typical. In the Pyrosomata 

 jixata it is broad and of variable form (plate-figs. 3, 4, and 8) ; on its dor- 

 sal edge is a flattened, pointed tentacle, which will be referred to as the 

 cloacal tentacle. In the other subgenus the aperture is usually a 

 circular pore (pi. 29) . 



The small, ovoid ganglion is located dorsally, near the anterior end 

 of the branchial basket. A pigmented layer in its ventral portion 

 probably enables some of the adjacent ganglion cells to perceive not 

 only the presence of light, but also its direction. From the sides of 

 the ganglion the nerves arise, eight pairs in all (fig. 5, pi. 17; fig. 13, 

 pi. 22) . Their distribution is characteristically different in the two 

 sub-genera. (See pp. 214 and 225.) Beneath the ganglion, closely 

 applied to its ventral and posterior surfaces, is the subneural gland, 

 and running forward between it and the ganglion is the duct which 



Fig. 4.— Outline drawings of the ganglion and neural gland: A,m Pyrosoma agassizi; B,in 



P. ATLANTICUM. X 193 DIAMETERS. 



connects it with the pharynx (fig. 4) . The duct in Pyrosoma is short 

 and not prominent. Its aperture, the ciliated pit, is oval or round. 

 It is directly beneath or often a little in front of the ganglion 



Circulatory system (fig. 1, pi. 15). The heart (h) lies at the poste- 

 rior end and a little to the right of the endostyle. As in other Tunicates, 

 the heart lies within a delicate pericardium, from which it originated 

 by dorsal, median invagination. The fold thus formed is closed along 

 its inner side by the endodermal wall of the pharynx. Each end re- 

 mains open, communicating with the blood sinuses. Of these sinuses 

 there are three principle ones, and several of secondary importance. 

 A median ventral sinus (v. s.), below the endostyle, is continuous in 

 front with a pair of peri-pharyngeal vessels (ph. s.), one running along 

 each of the peri-pharyngeal bands, and expanding laterally to contain 

 the cells of the luminous organ (I. o.) . These two sinuses unite around 

 the ganglion and continue along the dorsal side of the pharynx as a 

 median, dorsal sinus (d. s.) . The dorsal and ventral sinuses are con- 

 nected further by means of the transverse vessels of the branchial 

 lamellae, one in each trabecula. This connection is not a direct one, 



