26 THE VOYAGE OF 11. M.S. CHALLENGER. 



The Position. 



Before describing any part of the anatomy of an Ascidian, it is necessary to state the 

 position in which one considers its body placed, and to define such terms as anterior, 

 posterior, dorsal, and ventral ; as these have been used by some writers to denote 

 entirely different regions of the body. 



Savigny 1 placed his specimens, when describing them, in the natural position in 

 which most species are found, namely, with the apertures (or the branchial aperture when 

 they are far apart) superior, and the base of attachment inferior. He called the edge on 

 which the atrial aperture is placed the anterior region, and the opposite side, that next 

 the endostyle, posterior. Consequently, in all typical Simple Ascidians {e.g., Ascidia 

 mentula), according to Savigny's nomenclature of regions, the stomach and intestine 

 would lie on the right side of the branchial sac. 



Alder (1863), 2 Alder and Hancock (1870), and afterwards Hancock alone (1870), 3 

 employed a modification of Savigny's method. Their right and left sides were the same 

 as Savigny's, but they designated the region of the branchial aperture anterior, and the 

 base of attachment posterior. The two edges (anterior and posterior of Savigny) they 

 called dorsal and ventral. This was a distinct improvement, but the two last terms were 

 misapplied, the atrial edge being considered ventral, and the endostyle dorsal. 



Milne-Edwards, 4 like Savigny, placed the body upright, and called the branchial end 

 superior, and the place of attachment inferior. He differed from Savigny, however, 

 in considering the endostyle as anterior, and the atrial region as posterior, the result being 

 that his right and left sides are not synonymous with those of Savigny. 



Kupffer's 5 terms are partly those of Milne-Edwards. He calls the region of the bran- 

 chial aperture either superior or anterior, and the base of attachment inferior or posterior. 

 The edges, however, he designates as dorsal and ventral, applying these terms not as Alder 

 and Hancock did, but correctly — the atrium being dorsal, and the endostyle ventral. 

 The right and left sides thus come to be the same as those of Milne-Edwards. Heller's * 

 system is much the same ; he places the animal upright, and calls the branchial aperture 

 anterior, and the endostyle ventral. R. Hertwig 7 also considers the branchial aperture as 

 anterior or oral, and the base of attachment as posterior or aboral, whde the dorsal side is 

 indicated by the nerve ganglion, and the ventral by the endostyle. 



Lacaze-Duthiers 8 adopts an extraordinary system of nomenclature. He places the 



1 Mem. aiir les anim. sans Vert., part ii. fasc. 1. 



2 Observ. on Brit. Tun., Ann. Mag. Nat. Hist., ser. iii., vol. xi. p. 152. 



3 On the larval state of Molgula, &c, Ann. Mag. Nat. Hist., ser. iv., vol. vi. p. 353. 



4 Observ. sur les Asc. Comp., &c, Mdm. de VAcad. de Paris, vol. xviii. p. 217. 



5 Jahresberichte der Kommission zur Untersuchung der deutschen Meer in Kiel. Berlin, 1874. 

 r ' Untersuch. ii. d. Tun. des adriat. Meeres, Denhsckr. /.. Akad. Wissensck., Bd. xxxiv. 



7 Beitrage zur Kenntniss des Baues der Ascidien, Jen. Zeitschr., vol. viii. p. 74. 



8 Asc. Simp, des cotes de France, Arch Zool. expe'r. t. iii. 



