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, iii, 

 112). The endostyle 

 is always quite anterior 

 in position, and some- 

 times, as in Fig. 1 12, 

 removed by a consider- 

 able interval from the 



Fig. 112. — Diagram of tlie organisation of 

 Stigmata. ^ species of Appendlcularia, from the right side. 



In the posterior ex- '^'"-"'■"■'•''^'^'t''-*- , ,• ■, .„ 



^ a. Anus; tiie mdex line was accidentally 



tremity of the body drawn about '/» of an inch in front of the anus. 



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



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



male and female, in bacl<wards to the tail, passing to the right of the 



alimentary canal. .i.'.J. GlU-slit. /;. Heart, int. 



close proximity to one intestine, m. Mouth, n.c. Nerve-cord, with 



. . . ganglionic enlargements in the tail. o/. Otocyst; 



another, tine testis m beneath which the hypophysis opens into the 



front and the Ovarv branchial sac. cm. Ovary. f.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 



