Ascidian. 69 



with a dilated pharynx, but with the branchial cavity, and if their 

 inhalant aperture be taken to represent, not the mouth, but the 

 inhalant siphon of the Lamellibranchiata, it will be seen that the 

 essential character of the curves described by the digestive tract 

 beginning" with the orifice into which the upper end of the black 

 bristle is introduced, is the same as that of those described in the 

 Anodon, Preparation 19, p. 58, and that the two classes cannot be 

 contra-distinguished as ^ haemaP and 'neural^ respectively. The 

 digestive tract of the Ascidian is seen to differ from that of the 

 Anodon mainly in not possessing the two posterior of the three 

 concentric coils, described in that animal, whilst its first and most 

 important curve, and also that portion of it which is seen in the 

 cloaca, and in relation with the large blood-vessel passing from 

 the heart through the visceral mass to the ' branchiaF or ' dorsal 

 sinus ' of Milne-Edwards, corresponds very closely in all essential 

 particulars with the initial and the terminal segments respectively 

 of the digestive tract of the bivalve. The floor of the stomach is 

 occupied by a longitudinally folded eminence, which again is pro- 

 longed into and along the intestine, where it forms an elevated 

 ridge, dividing the tube into two demi-canals, and much increasing 

 its absorbing surface. The liver, which Mr. Hancock (Proc. Linn. 

 Soc, June 1867, p. 313), has, in opposition to Savigny, shown to 

 exist in the genus Ascidia, opens into the cavity of the stomach 

 by two ducts. The reproductive organs are seen in the concavity 

 of the second or intestinal ciirve of the digestive tract ; and their 

 excretory ducts open into the cloaca, together with the anus. 



The view which has been taken in this description of the rela- 

 tionship subsisting between the Lamellibranchiata and the Tunicata, 

 was sugg'csted by Cuvier in his memoir on the Ascidians (Memoires 

 du Museum^ 1815, torn, ii., p. 34), and was adopted by V. Baer, in 

 his paper on the route taken by the ova of Unionidae in passing 

 from the ovary into the branchial marsupium, already referred to, 

 and published in MeckeFs Archiv. for 1830, p. 341. This view 

 has been revived by Mr. Hancock (see Proceedings of Linnaean 

 Society, June 1867, p. 343), and illustrated by many new argu- 

 ments. 



Professor Huxley's view, according to which the branchial sac of 

 the Ascidian is to be regarded as a dilated pharynx, will be found 

 expounded by him in the British Association Report for 1H52, 



