62 
Tanks No. 3 and 4. 
The Scallop is one of the most widely known shells, the largest 
species, Pecten Jacobaeus (Fig. 107, Tank 3), having been used for ages 
by the pilgrims returning from the Holy Land, as a decoration of their 
hats and dress. The animal bears all round the margin of the mantle 
numerous short tentacles, and small beady eyes which shine like precious 
stones ; this can easily be seen when the shell is open. Their mode of 
progression is remarkable. They move about in jerks, by a rapid opening 
and closing of their shell, the hinge being behind ; when these contractions 
Fig. 106. Cardium aculeatwn, 
Y 2 nat. size. Tank 22. 
Fig. 107. Pecten Jacobaeus, 
V 2 nat. size. Tank 3. 
cease, they fall to the bottom again. The larger species have a flat and 
a concave shell, the latter is used for culinary purposes, to carry the 
"Ragout fin en coquille”. 
TUNIC AT A. 
(Tanks No. 4 and 20), 
Tank No. 4 presents a most curious picture of still-life. It contains 
groups of white couples of semi-transparent tubes; rough, knobbed masses 
like ground glass or wrinkled brown leather; or lastly red sack-like structures. 
Then we see green gelatinous masses, and on the sides of the tank are 
incrustations with delicate star-like markings. And all these curious forms 
scarcely reveal their animal nature; yet on closely observing them, the 
larger ones exhibit from time to time a closing and opening of their two 
large pores. They are called Sea-squirts or Ascidians. This apparent 
bed of curious plant-like growths is so completely misunderstood by the 
uninitiated, that we shall give a short account of their development and 
structure. 
Their most obvious characteristic is the "tunic” from which they 
derive their name; it is a protective covering, consisting of a sub¬ 
stance closely resembling the cellulose of plants (see p. 14). In Ciona 
(Fig. 109) it follows the shape of the animal, which is thus seen to consist 
mainly of two tubes, a longer and a shorter, bearing respectively the 
mouth and the outgoing pore. In Cynthia (Fig. 110) only the necks 
of these tubes project, while in Phallusia (Fig. 108) only their apertures 
are visible, the thick knobbed tunic giving an irregular outline. Food and 
oxygen are obtained in all the group by means of an almost continuous 
current (cp. p. 14) which enters the mouth and issues from the outgoing 
