AGGREGATED ASCIDIANS. SALP^. 431 



respiratory chamber. The individuals are occasionally connected 

 by a gelatinous flesh, like that which exists in many of the 

 compound Polypes ; and there is even sometimes found a calca- 

 reous deposition in this connecting substance, which would still 

 more closely establish their affinity with that group. It has been 

 recently shown, also, that all the individuals' in these compound 

 masses originate by germination, or budding, from a single 

 one. Both in the solitary and compound Ascidians, the young 

 animal, when it first issues from the egg, has active powers of 

 locomotion, being provided with a large tadpole-like tail, by the 

 stroke of which it is propelled through the water. Thus a pro- 

 vision is made for the general diffusion of these animals, which 

 would be otherwise crowded in particular spots. 



981. The Salpce are animals of greater delicacy of organisa- 

 tion than the greater part of the Ascidians. They are usually 

 so transparent, that the interior structure may be examined 

 through their tunic with little difficulty. They consequently 

 seldom possess any decided colour, except when viewed in a 

 bright sun-light ; and then they present splendid iridescent hues. 



Fio. 603. BIPHORA, OMB OF THK SALPJK ; a, mouth ; p, posterior or anal orifice ; /, liver, 

 inclosing other viscera ; c, heart ; br, branchial sac ; m, muscular bands ; n, nervous 

 ganglion. 



They have all one dark spot, however ; generally of a brownish- 

 yellow colour. This, which has been termed the nucleus, is 

 occasioned by the opacity of the liver, and its coloured secretion. 

 The branchial orifice possesses no tentacula; but these are 



GG2 



