:\K>ijj;sc'oniA. 



191 



fertilised ; but it is uncertain liow tlie fertilised ova escape into the 

 external medium. Besides true sexual reproduction, and besides 

 the power of producing colonies by continuous budding, fresh in- 

 dividuals can be pioduced in many cases by a process of discon- 

 tinuous genmiatitin. 



( 'lass II. Tl'NICATA. 



The members of this class are not uncommonly called Ascidian 

 Molluscs (Gr. askos, a, wine-skin), from the resemblance which many 

 of them exhibit in shape to a two-necked leather bottle (fig. 134, C). 

 They are popularly known as " Sea-squirts," frijm their power of 



Fig. 134. — MoiTjhology of Tnnio'ta. A, Dia^Taui of tlie structure of a simple Tunifate : 

 ( Test; t' Second muscular tunic ; .s Briirjchial .sac ; b Braitcliial ajierture; a. Atrial 

 aperture ; c Atrium ; o Opening of the gullet ; g Stomach, leatling into tlie intes- 

 tine; an Anal aperture; n Ncrve-gangliou. B, BoiniUus smurrrgd^i^, a "com- 

 pound " Tunicate— a small portion of a colony of the natural size, and a single 

 system of the same enlarged; co Common atrial aperture; hr Branchial a]ier- 

 tiu'P of one of the zoiiirls. C. MoJf/uJa Ulantiaitimsif, a simjile Ascidiaji. The 

 arrows in A and C show the .liri'ction of tin- water-cuiTCnts. 



forcibly ejecting water from the orifices of the bottle. Their scien- 

 tific name, again, of Tunirata, is derived from the fact that the 

 body is enveloped in a leathery elastic integument, which consists 

 of different layers, and which takes the place (.if a shell. The outer 

 covering of the animal is of a gristly or leathery consistence, and is 

 known as the^"test." It is reniarkal.ile for containing a con.sidera- 



