GENERAL NOTES ON TUNICATA. 453 
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 Diplosomide) the tailed 
larva is precociously reproductive, giving rise to buds before 
undergoing metamorphosis. This forms an_ interesting 
transition to the condition seen in /yrosoma, 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 Jarge 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 Sa/pa and 
Dotiolum, together with the aberrant deep-water 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 Sapa and Doléolum are pelagic in habit, and differ markedly in 
structure from the Ascidians. The body is fusiform (Sa/ga) or barrel- 
shaped (Do/olum), 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 Dolzolem, 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 
respiratory current as in the Ascidians, and the gill-slits may be few in 
number, or, as in Sa/pa, may be represented by two large holes in the 
walls of the pharynx. Further, the hermaphroditism is modified by the 
occurrence of very marked protogyny, and the ova are never numerous 
—in Sa/pa each sexual individual usually produces only one. 
On the other hand, the development exhibits marked alternation of 
generations, both solitary and colonial forms being included in one life 
history. 
In Doliobin the fertilised egg gives rise to a tailed larva, which 
develops into an asexual ‘‘nurse,” possessing the power of budding (cf. 
