534 Natural History of Molluscous Animals. 
water muscles, of the accuracy of which I am, however, by 
no means satisfied.* The scallops (Pécten), it has been 
asserted, can even leap by first opening their valves to the 
utmost, and then closing them by a strong and sudden effort. 
When deserted by the tide on any occasion, they will tumble 
forward in this way until they have regained the water. Nay, 
some popular writers repeat a story from the ancients that 
these scallops can rise up from their beds in the deep, and 
navigate the surface, having one valve raised and exposed 
with its concavity to the breeze, while the other remains 
under the water, and answers the purpose of an anchor, by 
steadying the animal, and preventing its being overset ; but 
this part of their history you may safely reject. 
There is lastly an order of rather doubtful Mollasca which 
have no shell, but merely a coriaceous membrane for their 
envelope, and which, in consequence, have been named Mol- 
lasca tunicata by modern naturalists. By far the greater 
proportion of these are fixed animals, but some of them swim 
in and on the ocean; not, however, by the aid of any particu- 
lar organ, but by partial contractions of their cloak. The 
Salpz are examples of this tribe, of which a number of indi- 
viduals belonging to the same species will cohere together by 
minute suckers which garnish their sides, and form floating 
chains, more obvious, it may be, in the night season than dur- 
ing the day, from the phosphorescent light they diffuse. The 
Pyrosdme (fig. 132.) are a still more singular family of the 
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same order. Each seeming individual of this genus is, in fact, 
a numerous colony of little Mollisca, each in its own cell, dis- 
tinct, yet inseparably connected with its fellows. Collected into 
the figure of a gelatinous cylinder, open at one extremity and 
closed at the other, and roughened externally by a multitude 
of tubercles disposed sometimes in rings and sometimes irre- 
gularly, they float in the Australian seas like stars of this 
lower world, shedding around them a halo of light, brilliant 
indeed, but surpassed in beauty by those other colours of the 
* Smellie’s Phil. of Nat. History, i. 137. 
