104 TUNICATA—SALPIANS CHAP. 
undergo laterai shifting, and rotation round their longitudinal 
axis, so as to acquire the relations seen in the “chain,” which 
then emerges from the tube in the test through which it has been 
growing, so as to project to the exterior near the atrial aperture. 
The buds at its free end which have now become far advanced 
in their development are set free in groups, which remain attached 
together by processes of the test, each enclosing a diverticulum 
from the body-wall (Fig. 62), so as to form “chains.” Each 
member of the chain is a Salpa of the sexual or aggregated form, 
and when mature may—either still attached to its neighbours or 
separated from them—produce one or several embryos (Fig. 61, 
A, emb), which develop into the solitary form of Salpa. Thus 
the two forms, different in appearance and structure and different 
in mode of origin, alternate regularly in the life-history of Salpa. 
Structure.—The more important points in the structure of a 
typical Salpa are shown in Fig. 65. The branchial and atrial 
apertures are at opposite ends of the body, and lead into large 
cavities, the branchial and peribranchial sac respectively, which are 
in free communication at the sides of the obliquely-running dorsal 
lamina or “gill” (dl). The transparent test is usually thick, 
and varies from a gelatinous to a stiff cartilaginous condition ; it 
adheres closely to the surface of the mantle (ectoderm and body- 
wall). The muscle-bands (from 4 to about 20—usually 8 
or 10) of the mantle do not in most cases completely encircle 
the body. They are present dorsally (Fig. 65, mus.bds) and 
laterally, but the majority do not reach the ventral surface. In 
many cases neighbouring bands join in the median dorsal line 
(Fig. 61). The muscle fibres are striated, and have rows of 
large equidistant nuclei. The anterior end of the dorsal lamina 
is in some cases prolonged to form a prominent tentacular organ, 
the languet or dorsal tentacle, projecting into the branchial sac, 
while near this opens a ciliated funnel corresponding to the 
dorsal tubercle, but having no connexion in the adult with 
either ganglion or subneural gland. The conjoined ganglion and 
subneural gland, the dorsal lamina, the peripharyngeal bands and 
the endostyle are placed in the usual positions. Eyes in the 
form either of a continuous horse-shoe-shaped pigmented ridge 
on the dorsal surface of the ganglion immediately below the ecto- 
derm, or of one larger median and several smaller lateral ocelli 
are found in the various species of Salpa. These eyes have in 
