20 . JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



Fig. 13. Seta from tentacular cirrus, showing septations, X 100. 

 Fig. 14. Eye showing two crystalline cones. 



Fig. 15. Posterior antenna showing sickle-shaped vestige of a seta 

 at a. X 100. 



Study III. — Anatomy and Life History of Salpa. 



The great heat of the past summer was apparently very favour- 

 able to the development of surface-life in British latitudes. Pelagic 

 animals, usually rare, were this year sometimes abundant. Thus 

 Beroe, Medusas and Copepoda of southern species, Tomopteris and 

 the pelagic Tunicates, were taken much more frequently than I have 

 ever known before. For several days during August the tow-net 

 was full of a transparent colourless barrel-shaped Salpa, which 

 proved to be S. mucronata-democratica, Forsk. Most were adult — 

 such as those figured on Plate II. were quite young, only a few hours 

 severed from the parent and still showing remains of a placenta. 



The large figures (16, 17 & 18) show this animal in various 

 aspects. In all the most striking features are seven great hoop- 

 shaped muscular bands. By comparing the three figures, the central 

 four will be seen to encircle the body completely, while the two 

 most anterior, together with the one most posterior, which lies 

 in front of the exhalent aperture (a), fail to complete their respective 

 circles, one upon the dorsal, two upon the ventral aspect (Fig. 17). 

 On the dorsal side (Fig. 16) lying between and connecting the first 

 two hoops, are a pair of band-shaped muscles whose function is to 



open the mouth. Another muscle of a — < shape (Fig. 18) lies 



horizontally on either side of the mouth, performing the opposite 

 function of closing its two lips. The remaining muscular bands are 

 two strap-shaped ones, lying between the angles of the exhalent 

 aperture (a). 



The body wall possesses externally a thick, semi-gelatinous layer, 

 which is the protecting tunic or test (t), formed originally of a glassy 

 homogeneous secretion from the ectoderm or primitive superficial 

 layer of the body. In this test are subsequently found numerous 

 cells which have wandered in from the ectoderm. These are specially 

 numerous in the extremities (t c). The mouth or inhalent aperture 

 (o) lies at the anterior end of the body ; the exhalent (a) at the 

 opposite end on the dorsal aspect. 



A great cavity is encompassed by the muscular hoops. An 

 anterior and a posterior division is formed by the presence of a 

 diagonal rafter or bar (6r), stretching from a point behind the 

 ganglion (g), backwards and downwards to the beginning of the 

 oesophagus. This bar has been supposed to be the much modified 



