ASCIDIA. 79 
by processes of the test, each enclosing a diverticulum 
from the body-wall, so as to form ‘‘chains.” Hach 
member of the chain is a Salpa of the sexual or agegre- 
gated form, and when mature may—either still attached 
to its neighbours or separated from them—produce one 
or several embryos, which develop into the solitary Salpa. 
Thus the two forms alternate regularly. 
Salpa, like Doliolum, is probably only an occasional 
visitor in our seas, but several species of the genus— 
Salpa democratica-mucronata, S. runcinata-fusiformis, 
and S. zonaria—have been found on occasions in the seas 
of the Hebrides, or cast ashore on our southern and 
western coasts. 
Order IIT. ASCIDIACHA. 
Fixed or free-swimming Simple or Compound Ascidians, 
which, in the adult, are never provided with a tail, and 
have no trace of a notochord. The free-swimming forms 
are colonies, the Simple Ascidians being always fixed. 
The test is permanent and well developed; as a rule, it 
increases with the age of the individual. The branchial 
sac is large and well developed. Its walls are perforated by 
numerous slits (stigmata) opening into the peribranchial 
cavity, which communicates with the exterior by the atrial 
aperture. Many of the forms reproduce by gemmation, 
and in most of them the sexually produced embryo develops 
into a tailed larva. 
The Ascidiacea includes three groups—the Simple 
Ascidians, the Compound Ascidians, and the free-swim- 
ming colonial Pyrosoma. 
Sub-order I. Asciprm@ SIMPLICES. 
Fixed Ascidians which are solitary and very rarely 
reproduce by gemmation; if colonies are formed, the 
