112 MOLLl'SCA. 
cular bands embrace tiie mantle and contract tlie body. The animal 
moves by taking in water at the posterior aperture, and forcing it out 
through that near the mouth, so that it is always propelled backwards, 
a circumstance which has led some naturalists into error by causing 
them to mistake the posterior opening for the true mouth * * * § . It 
usually swims on its back. The branchiee form a single tiibe or 
riband, furnished with regular vessels, placed obliquely in the middle 
of the tubular cavity of the mantle, in such a manner that it is con- 
stantly bathed by the water as it traverses that cavity f. The heart, 
viscera, and liver are wound up near the mouth and towards the 
back ; but the position of the ovary varies. The mantle and its en- 
velope when exposed to the sun exhibit the colours of the rainbow, 
and are so diaphanous, that the whole structure of the animal can be 
seen through them : in many they are furnished with perforated 
tubercles. The animal has been seen to come out from its envelope 
without appearing to suffer pain. The most curious circumstance 
respecting them, is their remaining united for a long time, just as 
they were in the ovary, and tlius swimming in long chains where the 
individuals are disposed in different ways, but each species always 
according to the same order. 
M. de Chamisso assures us, that he has verified a still more sin- 
gular fact relative to these animals ; it is, that the individuals which 
have thus issued from a multiplex ovary, are not fui’nished with a 
similar one, but produce insulated young ones of various forms, which 
have an ovary like that which produced their parent, so that there is, 
alternately, a generation of a few insulated individuals, and another 
of numerous and aggregate ones, and that these two alternating 
generations do not resemble each other 
It is verv certain that in some species little individuals have been 
observed adhering to the interior of large ones, by a peculiar kind of 
Slicker, which were dilferent in form from those that contained 
them §. 
These animals are very abundant in the Mediterranean and the 
Avarmer portions of the ocean, and are frequently phosphorescent. 
The THALiai, Brown, have a small crest or vertical fin near the 
posterior extremity of the back |i. 
* This has also happened to M. de Chamisso, in his Dissert, de Saljns, Berl., 
1819, and to others after him, but it is evident that there is no good reason for 
changing the denomination of parts in an animal merely because it swims on its 
hack, w’ith the head behind. It is thus that naturalists have been led into error 
with respect to the organization of the Pterolracheafa, which always swim on their 
back, a mode of natation common to numberless Gasteropoda both testaceous and 
naked. 
't' Some authors assert that this tube is perforated at both ends, and that the 
water traverses it ; I have endeavoured to convince myself of the truth of this 
assertion, but in vain. 
I Chamisso, loc. cit., I. p. 4. 
§ See my Mem. sur les Biphores, f. II. 
II Hvlothuria Thalia, Gm., Brown’s Jam., xliii, 3; — H. caudufa, Ib., 4; — II. 
deniidafa, Encyc. Method., Vers., l.xxxviii ; — Salpa critata, Cuv., Ann. du Mus., IV, 
Ixviii, 1, figured under the name of Dugysa by Home, Lect. on Compar. Anat. II, 
Ixiii ; — Salpa pinnata, Forsk., xxv, B. 
