562 



SEMIVERTEBRA TES. 



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 salpse. Among the 

 compound fixed types the colonies, as they are termed, consist of a number of 

 individuals produced 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. 1 



A LEATHERY SEA-SQUIRT, WITH ONE SIDE OF THE OUTER TUNIC REMOVED (nat. size). 



structure of Externally a simple sea-squirt, like the one (A. microcosmus) re- 



Ascidians. presented in the first illustration, has been aptly compared to a leather 

 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 external 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 composition 

 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 so-called atrial cavity, enclosing to a great extent the viscera, and com- 

 municating with the exterior by means of the exhalent orifice. The inhalent 

 orifice, or mouth, communicates, on the other hand, directly with the exceedingly 



1 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. 



