236 Description of the Plates. 



ventral, and the atrial chamber with the superior or dorsal com- 

 partment of the mantle cavity of the bivalve. The generative duct 

 is not figured in this diagram ; it opens however ordinarily, together 

 with the anus, into the free cloacal space which is seen between the 

 anus and the nerve ganglion^ and which corresponds very closely with 

 the space called '^ cloaca' in the description of Plate V; differing 

 indeed from it mainly in not being traversed by muscles such as 

 the posterior adductor and retractor. The vessels passing in the 

 non-cloacal portions of the atrial chamber from the pallial and 

 visceral plexuses through the tubular ties described by Mr. Hancock 

 to be distributed over the branchial net-work^ would, possibly, cor- 

 respond to the vascular system of the organ of Bojanus, which is 

 interposed between the systemic and the branchial vessels. The 

 absence of a foot^ and with it of a pedal ganglion, and the asymmetry 

 and less perfect evolution of the branchial sac in the Ascidians, must 

 be borne in mind as being the main points which mask the fun- 

 damental unity of type existing between the two classes under 

 comparison. 



For views more or less in accordance with those adopted here, 

 see Cuvier, Memoires du Museum, torn, ii., 1815, pp. 13-34, 

 or Memoires pour servir a THistoire et h. rAnatomie des 

 Mollusques, 1817; V. Baer, Meckel's Archiv., 1830, p. 341; 

 Hancock, Journal of Proceedings of Linnaean Society, June 

 20, 1867. 



See also AUman, ' Freshwater Polyzoa/ pp. 43—55 ; Quarterly Jour- 

 nal of Microscopical Science, Jan., 1869, p. 62 ; Nitsche, Dubois 

 Reymond und Reichert's Archiv., 1868, Hft. iv., p. 518; La- 

 caze Duthiers, Ann. Sci. Nat., Ser. v., tom. iv., 1865; Sur un 

 genre nouveau d'Ascidian Le Chevreulius Calleusis, L.D. 



For the view according to which the branchial sac of the Ascidian 

 represents the dilated pharynx of a Lamellibranchiate Mollusc, 

 see Milne Edwards, Observations sur les Ascidies Composees, 

 Memoires de Tlnstitut, xviii., p. 274; Huxley, Phil. Trans., 

 1 85 1 ; British Association Reports, 1 852 ; Transact. Linn. Soc, 

 i860, xxiii., p. 202. 



For the structure of the external test, see Huxley, Cyclopaedia of 

 Anatomy and Physiology, article ' Tegumentary Organs ;' F. E. 

 Schviltze, Zeitschrift fiir Wiss. Zool., xii., p. 175, 1863. 



