ANATOMY AND DEVELOPMENT. 



237 



larval condition of the fixed Ascidians, that they retain the 

 tail as their organ of locomotion throughout life (Fig. 1 1 1). 



The tail is inserted in the middle of the ventral surface 

 of the body proper, and is obviously a mere appendage of 

 the latter. 



The mouth is terminal or sub-terminal. There is a sin- 

 gle pair of branchial stigmata, which open into a pair of 

 tubular atrial cavities, whose separate external apertures 

 are seen in front, on the ventral surface behind the mouth. 



The alimentary canal is U-shaped, and the anus opens 

 on the ventral surface to the right of the middle line, some- 

 times behind and some- 

 times (according to the 

 species) in front of the 

 stigmata (Figs, in, 

 112). The endostyle 

 is always quite anterior 

 in position, and some- 

 times, as in Fig. 112, 

 removed by a consider- 

 able interval from the 

 stigmata. 



tic 



Fig. 112. Diagram of the organisation of 

 a species of Appendicularia, from the right side. 



In the posterior ex- < After HERDMAN.) 



a. Anus ; the index line was accidentally 



tremity Of the body drawn about % of an inch in front of the anus. 



... b.s. Branchial sac. ch. Notochord. e. Endostyle. 



are placed the gOnadS, ^ Ganglion, from which the nerve-cord proceeds 



male and female, in backwards to the tail, posing to the right of the 



alimentary canal, g.s. Gill-slit, h. Heart, tut. 



close proximity tO One Intestine, m. Mouth. n.c. Nerve-cord, with 



, , . . ganglionic enlargements in the tail. of. Otocyst ; 



another, the tCStlS in beneath which the hypophysis opens into the 



front and the OVarV branchial sac - ov - Oval 7- P- b - Peripharyngeal 



* band. st. Stomach, te. Testis. 



behind. The heart, as 



described by LANKESTER, is a unique example of a func- 

 tional organ reduced to the lowest possible level of histo- 

 logical structure. It consists simply of two cells placed 



