BIRD'S-HEAD APPENDAGES. 129 



shut like a pair of pincers. Each head is fixed on 

 a flexible stalk, and while the creature lives, main- 

 tains a constant and regular motion up and down, 

 opening and shutting the bill at 

 intervals. All specimens of the 

 same species do not produce them, 

 nor are they found on all the cells 

 of a single specimen ; and they 

 exist indiscriminately on certain 

 species of different genera, while 

 often species otherwise closely al- 

 lied are not furnished with them. 



From this group of the class Zoophytes we pass, by 

 a very easy transition, to the more simple members of 

 the MOLLUSCA, those forming the subdivision tunicata. 

 They are so called, because their soft parts are enclosed, 

 not in a shell, like the majority of the class, but in a 

 tough, leathery coat or tunic. The commonest example 

 of a tunicated Mollusc is found in the various kinds 

 of Ascidice, or Sea Squirts, some of which are found 

 attached to sea-weeds and stones, in the littoral zone ; 

 others are frequently thrown up from deeper water 

 on the beach, and may be dredged in abundance in 

 almost any locality. Some are of a large size, several 

 inches in length. Their outer form is that of a bag, 

 with a smooth or variously-roughened semi-transpa- 

 rent skin, furnished with two small openings, through 

 which, on the slightest pressure, a jet of water is sent 

 to a considerable distance. These creatures lead a very 

 inactive life. Attached by their base to plants, they 

 trust for nourishment to whatever small fry are brought 



