4 Part second. 



of the ocean, -the Cockle and Scallop are by no means slow of movement. 

 The Cockle, Cardium (Fig. 154), is a genus comprising many species; 

 it derives the Latin name from the form of its shell. The animal has 



Fig. 172. Solen vagina, i/ 2 nat - size, on the left the foot, on 

 the right the respiratory tubes. 



a long, bent foot of a brilliant red colour, and knows how to make good 

 use of it. It presses it on the ground, stiffens it out by forcing blood 

 into it, and in this way clears a distance of several feet; this jumping 

 seems a most surprising movement for a shell to make, but it is not 

 solely restricted to the Cockles. It can also burrow very rapidly in the 

 sand, using the bent foot as a hook. It is a very toothsome shell-fish 

 and is collected in enormous quantities on the British coasts. 



The Scallop is one of the most widely known shells, the largest 

 species, Pecten jacobaeus (Fig. 24), 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 open- 

 ing and closing of their shell, the hinge being behind; when these con- 

 tractions 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". 



TUNICATA. 



Tank Nr. 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 observ- 

 ing them, the larger ones exhibit from time to time a closing and open- 

 ing of their two large pores. They are called Sea-squirts or Ascidians. 

 This apparent bed of curious plant-like growths is so completely misun- 

 derstood 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. 54). In Ciona 

 (Fig. 28) it follows the shape of the animal, which is thus seen to con- 

 sist mainly of two tubes, a longer and a shorter, bearing respectively the 

 mouth and the outgoing pore. In Cynthia (Fig. 30) only the necks 



