332 M. Ussow's Zoohgico-Emhryological Investigations. 



Among tlie subsidiary organs of the alimentary apparatus 

 I reckon also an enigmatical glandular organ, detected by me 

 in some simple {At>cidia iidestinaUs^ cdniini, Cynthia niicro- 

 cosjmts) and social {Clavellina lepadiformis) Ascidia. Tliis 

 sac-like gland, consisting of two coalescent portions, is situated 

 sometimes Inneath'^j sometimes above'\ the central ganglion. 

 It consists of numerous, very variously bent, can-al tubes, lying 

 within a common envelope. Their cavities, which are lined 

 with simple cylinder-epithelium, contain spherical bodies of 

 various sizes. All the tubes of each half of the gland unite in 

 its centre to form a more or less thick tube, or an eftcrentduct, 

 which opens into one of the nearest cavities of the ciliated pit. 

 This gland occurs very early in young Ascidia ; but I have 

 not succeeded in ascertaining from which germ-lamella, or 

 from the parts of which organ, it is formed (by e version ?), 



The Tunicata are not Mollusca. Even without taking into 

 consideration the mode of embryonal development, a com- 

 parison of the plan of structure of the different ^lollusca with 

 that of the Tunicata suffices to refer the latter witli more 

 propriety to the Vermes. The simple cardiac tube, the absence 

 of the oesophageal ganglia and their commissure, the complete 

 absence of the foot, the curvatures of the intestinal canal 

 directed towards the heart, the existence of the outer mantle, 

 and the peculiarities of its structure, mode of formation, and 

 chemical constitution, the variability in the directions of the 

 contractions of the cardiac tube, &c., all draw a more or less 

 sharp boundary-line between the Timicata and the Mollusca. 

 The Tmiicata approach most closely to the BryozoaJ. 



On the other hand, it must be admitted that the simplicity 

 in the stiiicture of the nei-vous system (the Appendicidarice 

 excepted) and the cardiac tube, the relation of the respiratory 

 organ to the upper part of the intestinal canal (Balanoglossus) , 

 the indistinct separation of the inner mantle from the muscular 

 layer (denno-muscular layer), and the verygenei-al alternation 

 of generations, constitute characters by which the Tunicata in 

 some degree approach the type of Vermes § (to which, thanks 



* In Ascidia intestiiialis and canina and Clavellina lepadiformis. 



+ In Cynthia microcosmtis. 



X Huxley, Lect. on Comp. Anat. pp. 80, 85 ; Iliickel, Gen. Morphol. 

 Bd. ii. p. ciii ; Albnan, Brit. Assoc. litp. 1850, and Trans. Hoy. Irish 

 Acad, vol, XX. 1852, p. 275 ; Van Beneden, M^m. Acad. Belg. vol. xx. 

 pp. 54-58. 



§ Gegeubaur, Vergl. Anat. 2te Aufl. p. 128 ; Ilackel, Natiirl. Scbop- 

 fungsgesch. 4te Aufl. pp. 448, 406, 4G7. 



