540 Vertebrata. Appendix : Tunieata. 



tube, their cloacal apertures into the cavity ; the water which is 

 taken in at the mouth thus passes into the tube, from the open end 

 of which it is expelled; by means of this exhalent current the 

 colony is driven through the water, with the closed end forwards. 



The free-swimming Salpa offers a remarkable modification of 

 the Ascidian type. The buccal and cloacal apertures are almost at 

 opposite poles of the body. The branchial sac is, however, very 

 degenerate ; its lateral walls are absent, so that, with the exception 

 of the ventral region with the endostyle, only the dorsal portion 

 remains as a band stretching across the united branchial and peri- 

 branchial cavities (r. Fig. 427). The visceral mass is of insignificant 

 size as compared with that of the whole animal, the cavity just 

 mentioned constitutes the greater part of the animal (in most 

 Tunieata the branchial cavity is very extensive) . In the transparent 

 body-wall, beautiful circular muscle-bands are seen, by the contrac- 



Pig. 427. Diagram of a Salpa, 

 a anus, cl cloacal opening, k 

 branchial cavity, i gut, m muscle 

 bands, o mouth, p peribranohial 

 cavity, r dorsal lamina. — Orig. 



tions of which water is expelled; they are homologous with a 

 continuous layer of muscle which is present in the body-wall of 

 Ascidia. The Salpidse are not only remarkable in structure, but 

 also in affording an instance of a regular alternation of 

 generations. There are both solitary asexual forms 

 and chains consisting of a larger or smaller number of somewhat 

 loosely connected sexual individuals. The asexual generation 

 forms chains by budding; the chains remain within the body- wall 

 of the solitary salp until they have attained a certain development, 

 when they break free and swim about independently. The solitary 

 and colonial individuals differ somewhat from each other. The 

 colonial forms are remarkable in that they first give rise to eggs 

 which are fertilised by spermatozoa from another chain, whilst later 

 on they themselves produce spermatozoa. Each individual usually 

 produces only a single egg, which undergoes development in the 

 body of the parent. 



Of tlie forms mentioned atove, vai-ious species of simple Ascidians are 

 widely distributed in Eui'opean seas ; they are attached to seaweed, stones, piles, 

 etc. There are species of CompositsB and of Appendicularia, also in 

 northern seas ; Pyrosoma and Salpa are pelagic, and occur in the large 

 oceans as well as in the Mediterranean. All the Tunieata feed upon microscopic 

 organisms, which they take into the branchial cavity with the water. 



