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THE VOYAGE OF H.M.S. CHALLENGER. 
of the post-abdomen, and the remainder was much less opaque than usual. The ova are 
found in a small group, usually placed near the anterior end (see PI. XXV. fig. 6, o.), 
and are united by narrow tubular pedicels. 
The spermatic vesicles are pyriform and of small size. The vas deferens is narrow 
but distinct, and may be traced from the post-abdomen forwards through the abdomen 
and thorax, where it lies alongside the rectum (see PI. XXY. fig. 6, v.d.). 
Polyclinum, Savigny. 
Polyclinum, Savigny, Memoires, 1816. 
Polyclinum, Milne-Ed wards. Observations, &c., 1842. 
Polyclinum, Forbes, British Mollusca, voL L p. 14, 1853. 
Polyclinum, Giard, Eecberches, &c.. Archives de Zool. exp4r., vol. i. p. 641, 1872. 
Polyclinum, von Drasche, Die Synascidien, 1883. 
Colony massive, usually sessile. 
Systems simple or compound, often irregular. 
Ascidiozooids elongated, more or less distinctly divided into three regions ; 
branchial aperture six-lobed ; atrial aperture provided with a languet. 
Test gelatinous or cartilaginous, sometimes incrusted with sand. 
Branchial Sac large and well developed. 
Alimentary Canal usually long and complicated, often twisted ; stomach smooth 
walled. 
Post- Abdomen separated from the abdomen by a distinct constriction, often 
projecting from one side of the intestinal loop. 
Savigny in characterising this genus laid stress upon the distinct but often irregular 
systems, each provided with a common cloaca, the six lobes of the branchial aperture and 
the well-developed atrial languet, the large branchial sac, and the distinct division of the 
body of the Ascidiozooid into three regions. These characters, however, are not sufficient 
to distinguish the genus from some of the other Polyclinidse, and consequently subsequent 
authors have added to and modified them considerably. Giard pointed out that in 
many species of Polyclinum the alimentary canal is so twisted that the rectum crosses 
over the right side of the stomach. He considered this disposition of the intestinal 
loop as the most important character of the genus, and one which distinguished it from 
all the other Polyclinidae. Yon Drasche, however, found it necessary to modify Giard’s 
definition slightly, so that Polyclinum might include forms where the alimentary canal 
was so placed that the stomach lay not on the left side of, but anteriorly to, a part of the 
rectum. I have met with both of these arrangements and also with others which seem 
to show a gradual transition between the twisted condition found in Savigny’s species ^ 
1 See bis Memoires, pis. xviii. and xix. 
