278 



ACRAiJIA. 



TuNicATES (figs. 144, 145) or Sea-squirts are flask- or bottle- 

 sliaped gelatinous-looking masses found attached to rocks, &c., in 

 the sea. That they constitute an important group we can readily 

 understand when a monograph of the forms found hy the 

 Challenger expedition reaches eight hundred odd quarto pages. 

 Such subjects are outside our province, as they are purely 

 marine animals. It is sufficient for our purpose to point out 



Fig. 144 —Structure of a Tunicate. 



A, Diagram of Hie structure of a simple Tunicate : (, test ; f, second muscular tunic ; 

 s, branchial sac ; h, branchial aperture ; a, .atrial opening ; c, atrium ; o, opening o; 

 gullet ; g, stomach ; an, anus ; n, nerve-ganglion, b, Botryll'us sniorai/di/s— portion of 

 a colony : co, common atrial pore ; &r, hrancliial aperture of one of the zotiids. c, a 

 simple Ascidian (Molgtda). (Nicholson.) 



that these Sea-squirts, which in their adult form are undoubtedly 

 invertebrate in type, are in their larval or early stages distinctly 

 vertebrate. The so-called Appendicularia larva (fig. 145, a) has 

 a distinct dorsal skeletal rod, the notochord (7), in its tail, which 

 entirely disappears in the adult, — the dorsal nervous system 

 becoming ventral by a curious and complicated series of 

 changes. 



