84 Part second. 



While all the above named bivalves afford examples ot the still-life 

 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, '/a 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 and French coasts. 

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

 species , Peeten jaeobaeus (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; these 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 « Ragoiit fin en coquillc ». 



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 

 structure and development. 



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 Ciena 

 (Fig. 30) it follows the sliape of the animal , which is thus seen to con- 

 sist mainl.y of two tubes , a longer and a shorter , bearing respectively 

 the mouth and the outgoing pore. In Cynthia (Fig. 28) only the necks 



