434 SUB-PHYLUM UROCHORDA OR TUNIC AT A. 



Both processes are carried further in the compound 

 Ascidians. In these the eggs have usually a considerable 

 amount of yolk, and development takes place either in the 

 atrial cavity of the mother, or in special brood-pouches. In 

 consequence, the development, especially in the early stages, 

 shows considerable modification, although the larval stage 

 is quite distinct. Again, the tailed larva develops into an 

 adult which has no sexual organs, but forms a colony by 

 budding. The individuals of the colony then give rise to 

 eggs and so to larvae. The development thus includes a 

 distinct alternation of generations. 



Budding takes place in many different ways in the com- 

 pound Ascidians. In one set (the Diplosomidae) the tailed 

 larva is precociously reproductive, giving rise to buds before 

 undergoing metamorphosis. This forms an interesting 

 transition to the condition seen in Pyrosoma, where the 

 fertilised egg gives rise to a rudimentary larva (cyathozooid), 

 from which a young colony of four individuals arises by 

 budding. These individuals again bud, until a large colony 

 is formed, the members of which become sexual. The ova 

 are few in number, a statement which is generally true for 

 the pelagic Tunicates, as contrasted with sedentary forms. 



While the Ascidians in the narrow sense include all the 

 more typical Tunicates, there are two other sets, few in 

 number both as regards genera and species, but of great 

 theoretic importance. 



The one set includes the free-swimming genera Salpa and 

 Doliolum, together with the aberrant Challenger genus 

 Octacnemus ; the other, a few active free-swimming forms, 

 which exhibit throughout life many of the characteristics of 

 the larval Ascidian. Of these, Appendicularia is the most 

 familiar type. 



Both Salpa and Doliolum are pelagic in habit, and differ markedly in 

 structure from the Ascidians. The body is fusiform (Salpa] or barrel- 

 shaped (Doliolum}) and wholly or partially encircled by definite muscle 

 bands, which replace the scattered fibres of the Ascidians. The mouth 

 is at one end of the body, and the atrial aperture at the other ; the 

 animals swim by forcing the water out of the peribranchial chamber 

 posteriorly. Many of the most marked signs of specialisation in the 

 Ascidians are here absent. Thus the test may be, as in DoUolum, very 

 thin and devoid of cells, and the branchial sac is relatively simple in 

 structure ; the cilia on its walls are never so important in producing the 



