TUNICATA. 27 



The young animal in this Class passes through 

 a metamorphosis. At first it has a tadpole-like 

 form, with a long flattened tail, which is an efficient 

 swimming organ. Button-like warts bud out from 

 the fore part, by which the infant Tunicate pre- 

 sently cements itself to some fixed body; soon 

 after which the tail is absorbed, and the adult form 

 is assumed. 



The nervous system is represented by a single 

 ganglion, situated beneath the mantle between the 

 two orifices. Nerves pass from this centre to various 

 parts of the body, and to the organs of sense. The 

 most distinct of these latter are the eyes and the 

 tentacles. In many species we see eight red specks 

 placed around the margin of the receiving orifice, 

 and six around the discharging one. These are 

 eyes, which possess (according to Krohn and other 

 anatomists) all the parts necessary for distinct 

 vision, viz., the sclerotic coat, cornea, pigment, 

 iris, tapetum, vitreous humour, crystalline lens, 

 retina, and optic nerve. 



Fixed and almost motionless as most of the 

 TuNiCATA are, during life, the muscular system 

 is little developed in them. In general it consists 

 of a few circular and longitudinal bands, the chief 

 operation of which is the spasmodic enlargement 

 or contraction of the body-cavity at certain inter- 

 vals, by which the surrounding water is emitted or 

 ejected. The opening and closing of the orifices 

 are, in most cases, the only voluntary movements 

 that can be detected in these sluggish animals. 



Some modifications under which the TuNiCATA 

 present themselves to us require to be noticed. 

 Many of the forms are compound, such as the 

 Botrylli; those little patches of firm jelly which we 



