THE SEA SQUIRTS OR 'ASCIDIANS 



2939 



perpetually in the open ocean, and have become adapted in the extremest manner to 

 a purely pelagic environment." As there are both simple and compound fixed 

 ascidians, so there are two similar types among the pelagic forms; but some of the 

 latter are complicated by an alternation of generations, the one generation being a 

 simple form, whereas in the other generation the units are aggregated into chains, 

 as shown in our plate of the creatures known as salpae. Among the compound 

 fixed types the colonies, as they are termed, consist of a number of individuals pro- 

 duced by budding from a single parent stock; such colonies frequently attaining 

 very large dimensions, and being remarkable for their brilliant coloration, although 

 in other cases they merely form thin incrustations on the surface of various marine 

 objects. Other forms, on the contrary, are merely connected at their bases by a 

 common creeping root-like base, from which new buds are from time to time given 

 off, the individuals being otherwise free.* 



A LEATHERY SEA SQUIRT, WITH ONE SIDE OF THE OUTER TUNIC REMOVED. 



(Natural size.) 



Externally a simple sea squirt, like the one (A. microcosmus) repre- 

 sented in the first illustration, has been aptly compared to a leather 

 Ascidians 



bottle with two spouts; these spouts forming funnel-shaped projections, 

 one of which generally situated at a higher level than the other takes in water, 

 which is discharged from the second. The whole organism is invested in an ex- 

 ternal tunic, varying much in structure, but being frequently warty, and generally 

 opaque, although in the salpse it is transparent. A remarkable feature connected 

 with this outer tunic is that it contains a substance cellulose identical in compo- 

 sition with that forming the cell walls of plant tissues. On cutting through the 

 outer tunic, we come, as in our second illustration, to an underlying muscular tunic, 

 forming the true body wall, and consisting externally of an epidermis underlain by 

 interlacing muscular fibres. In the illustration, a indicates the inhalent, and b the 

 exhalent orifice of this inner tunic. On cutting into the inner tunic, we find a large 



*Strictly speaking, the term " individual " includes all the units produced by budding from a common stock, 

 but it is more convenient to use it in the ordinary sense. 



