104 TUNICATA SALPIANS CHAP. 



undergo lateral 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, em&), 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 " (d.l}. 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-sha,ped 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 



