MOLLUSCOIDA. 



191 



fertilised ; but it is uncertain how the 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 produced in many cases by a process of discon- 

 tinuous gemmation. 



Class II. Tunicata. 



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," from their power of 



Fig. 134.— Morphology of Tunicata. A, Diagram of the structure of a simple Tunicate : 

 t Test ; t' Second muscular tunic ; s Branchial sac ; h Branchial aperture ; a Atrial 

 aperture ; c Atrium ; o Opening of the gullet ; g Stomacli, leading into the intes- 

 tine; an Anal aperture; n Nerve-ganglion. B, Botryllus smo.ragdvs, a "com- 

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

 system of the same enlarged ; eo Common atrial aperture ; 67- Branchial aper- 

 ture of one of the zooids. C, Molgula ManJmttensis, a simple Ascidian. The 

 arrows in A and C show the direction of the water-currents. 



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

 tiiic name, again, of Tunicata, 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 of a shell. The outer 

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

 known as the " test." It is remarkable for containing a con.sidera- 



