240 



MANUAL OP ZOOLOGY. 



CHAPTER XLIII. 

 TUNIC ATA. 



CLASS II. TUNICATA (Ascidioida). The members of this class 

 of the Molluscoida are defined as follows : ' Alimentary canal 

 suspended in a double- walled sac, but not capable of protrusion 

 and retraction; mouth opening into the bottom of a respi- 

 ratory sac, whose walls are more or less completely lined by a 

 network of bloodvessels.' (Allman.) Animal simple or com- 

 posite. An imperfect heart in the form of a simple tube open 

 at both ends. 



The Tunicaries are all marine, and are protected by a lea- 

 thery, elastic integument, which takes the place of a shell. In 

 appearance a solitary Ascidian (fig. 74) may be compared to 

 a double-necked jar with two prominent apertures situated 



d 



Fig. 74. Morphology of Tunicata. 1. Diagram of aTunicary (after Allman) ; 

 a. Oral aperture; b. Atrial aperture; c. Pharyngeal or branchial sac, 

 with its rows of ciliated apertures ; d. Alimentary canal, with its haemal 

 flexure ; e. Anus ; /. Atrium ; g. Nervous ganglion. 2. Cynthia papillosa, 

 a simple Ascidian. (After Woodward.) 



close to one another at the free extremity of the animal, one 

 of these being the mouth, whilst the other serves as an excre- 

 tory aperture. The covering of an Ascidian is composed of 

 two layers. Of these the outer is called the 'external tunic,' 

 or * test,' and is distinguished by its coriaceous or cartilaginous 

 consistence. It is also remarkable for containing a substance 

 which gives the same chemical reactions as cellulose, and is 



