170 



Trajisactious. 



The iuternal lougtitudinal bars are much closer on the folds than in the 

 interspace, and the meshes are square on the folds, but oblong transversely 

 in the interspace. The transverse vessels sometimes branch and make 

 parts of the sac appear rather irregular. These vessels also vary consider- 

 ably in width ; in some cases they are as wide as the meshes, and vary 

 from this extreme to slender vessels, which may, though very rarely, im- 

 perfectly divide two meshes. Sometimes many wide vessels are present, 

 but usually several narrower ones appear between the widest (fig. 1). The 

 number of stigmata in a mesh also varies greatly, 6 up to 12 or 15 being 

 commonly found. On the folds there are 4 to 6 in a mesh. The stigmata 

 are wide and clearly defined, and are lined by cilia. 



The endostyle is prominent ; the groove itself is narrow, and anteriorly 

 joins the well-marked perioesophageal ring. 



The dorsal lamina is a plain broad membrane, slightly puckered, and 

 generally is found bent upwards to the right, forming a groove. In one 

 specimen examined this groove contained a strand of what was apparentl}' 

 food material in the act of passing from the lamina into the oesophagus. 



Fig. 2. Fig. 3. 



Fig. 2. — Part of the ring (jf tentacles of Styela coerulea ; X 13. /, t', t", long, medium. 



and short tentacles respectively. 

 Fig. 3. — TioTfisA twhwcXe oi Styela coerulea ; x 25. ci, dorsal lamina ; p> perioesophageal 



tract ; s, slit. 



The tentacles (fig. 2) are simple, and, on an average, number 80. Roughly, 

 three sizes may be recognized : (a.) The largest extend almost to the centre 

 of the siphon ; of these there are about 20. {h.) The medium-sized ten- 

 tacles also number about 20, and alternate with the largest, (c.) The 

 smallest tentacles, each of which lies between an (a) and a {h), number 

 about 40. 



The dorsal tubercle (fig. 3) is circular in outline, except for a slight inden- 

 tation on the anterior surface. The slit is S-shaped, the left limb having 

 a wider curve than the right. I have found this structure fairly constant ; 

 very few of the specimens examined showed a different form of slit. 



The alimentary canal (fig. 4) is situated in the posterior half of the body, 

 and is attached to the left side. It forms a narrow loop, the bend of which 



