GENERAL NOTES ON TUNICATA. 455 
the sexual generation, but only a few become truly sexual. The two 
.. lateral series develop into nutritive forms, which supply the nurse with 
food. The nurse itself loses its alimentary and respiratory organs, and 
becomes a mere organ of locomotion. The median buds develop into 
‘* foster mothers,” which ultimately go free, bearing with them other 
buds destined to develop into the solitary sexual forms. In these, first 
ova and then spermatozoa are produced, which start the life cycle afresh. 
It is thus obvious that there is considerable division of labour in the 
sexual form, accompanied by polymorphism ; the whole process presents 
some curious analogies to the conditions seen in the Ccelentera, 
In Sala the single egg is fertilised within the body of the mother, 
and becomes attached to the wall of the peribranchial chamber. Here 
the developing egg is nourished by means of a “placenta,” and the 
development is in consequence much abbreviated, the tailed larva not 
being represented. This embryo gives rise to a solitary ‘‘nurse” form, 
ana aon 
SAU UY \ 
Fic. 246.—Diagram of Salpa africana. 
o.a., Oral aperture ; @.¢., dorsal tubercle; Ze., tentacle; g., ganglion; ., 
muscle bands; ady., atrium; &.v., blood-vessel; az., anus; a@.a., 
exhalant aperture; v.7., visceral nucleus; 4., heart; s¢., stolon; 2.2, 
dorsal lamina; Z., endostyle; s.2.g., sub-neural gland; 2%., pharynx ; 
p.p.., peri-pharyngeal band. 
which by budding produces a chain of embryos. This chain is set free, 
jts members become sexual, and, either while still united or after 
separation, give rise to the eggs which develop into the nurse form, 
The remaining order of Tunicates includes minute simplified forms 
like Appendicularia, also pelagic in habitat, but without any power of 
budding, and never forming colonies. These forms have a distinct tail, 
which is bent at an angle to the body, and is the main-organ of locomo- 
tion. The mouth is at the anterior end; the anus, which is distinct 
from the atrial openings, is at the root of the tail. These atrial openings 
lie slightly behind the anus, and are merely small ectodermic invagina- 
tions communicating with the two gill-slits of the pharynx. They 
correspond to the similar invaginations in the Ascidian larva. The test 
may form a large investing ‘‘ house,” but it does not contain cells, and 
is periodically cast and renewed. The important points as regards 
internal structure are the presence of the notochord throughout life, and 
the structure of the nervous system. The latter consists of a lobed 
