66 Descriptions of Preparations. 



branchiata, and in the Anodon particularly, see Duvernoy, 

 Memoires de l^Institut, torn. xxiv._, 1854, and pi. 7, fig. 2, 

 pi. 8 and 9, fig-s. i and 2, and pp. 87-96 for that of the 

 Naiades. 

 For a more general account, see English Cyclopaedia, Article 

 'Mollusea/ p. 869; SiebokFs Comparative Anatomy, Ame- 

 rican Translation, p. 198 ; Gegenhaur^s Vergleichende Ana- 

 tomic, p. 310, with excellent figures 77, A. B. C, pp. 315, 316; 

 Hancock and Embleton, Phil. Trans. 1852, p. 239. 



22. AsciDiAN (Ascidia Affinis). 



Dissected and prepared similarly to Preparation 19, and showing thus the homolo- 

 gical relations of the several structures of the Tunicata to those of the Lamelli- 

 branchiata. 



The animal has been suspended by what was, in its erect position 

 during life, its base of attachment ; and its inhalant and exhalant 

 orifices point, consequently, downwards instead of upwards. Parts 

 of the test, of the two internal tunics, and of the upper part of the 

 branchial sac, have been removed on the animaFs right side, which 

 corresponds with the front of the preparation ; and so much also of 

 the walls of the stomach, and intestine, and of the vesicular substance, 

 and of the liver in connection with them, has been similarly taken 

 away, as was necessary for exposing the interior of the digestive 

 tract from the mouth to the anus. A black bristle has been intro- 

 duced through the inhalant orifice along- the interior of the branchial 

 sac into the mouth and stomach, and a white one has been simi- 

 larly passed through the exhalant orifice into the rectum. In this 

 species the inhalant orifice is terminal, and the anal is lateral; 

 just as in many Lamellibranchiata the inhalant siphon, or the 

 tentaculate portion of the mantle which corresi^onds to it in the 

 non-siphonate orders, reaches farther backwards than the anal 

 siphon, or the anal portion of the mantle. The mantle, which is 

 recognisable by its muscular fibrillation, has since the death of the 

 animal shrunk away from the external test, the homologue of the 

 bivalve shell. The rectum and the generative ducts are seen to open, 

 as in life, a little way within the periphery of the mantle, and into 

 a space which is homologous with that described. Preparation 21, 



