MOLLUSCOIDA: TUNICATA. 287 



Sub-order 6. Cyclostomata (marine). 



Cell tubular J orifice terminal, of the same diameter as the cell, 

 without any movable apparatus for its closure ; consistence cal- 

 careous. 

 Sub-order 7. Ctenostomata (marine). 



Orifice of the cell terminal, furnished with a usually setose fringe 

 for its closure ; cells distinct, arising from a common tube ; consist- 

 tence homy or camose. 



CHAPTER XLIII 



TUNICATA. 



Class II. Tunicata (Ascidioidd). — The members of this class 

 of the Molluscoida are defined as follows : — " Alimentary canal 

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

 sion and retraction ; mouth opening into the bottom of a re- 

 spiratory sac, whose walls are more or less completely lined by 

 . a network of blood-vessels." — (Allman.) Animal simple or 

 composite. 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. 103) may be compared to 

 a double-necked jar with two prominent apertures situated 

 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 

 probably identical with this characteristic vegetable product. 

 The test is lined by a second coat, which is termed the " second 

 tunic," or " mantle," and which is mainly composed of longi- 

 tudinal and circular muscular fibres. By means of these the 

 animal is endowed with great contractility, and has the power 

 of ejecting water from its branchial aperture with considerable 

 force. , The liiantle lines the test, but is only slightly and 

 loosely attached to it, especially near the apertures. The 

 mouth is generally surrounded by a circlet of small, non-ciliated, 

 non-retractile tentacles, and opens into a large chamber (fig. 

 103, I, c), which usually occupies the greater part of the cavity 



