416 THE MICROSCOPE. 



Tunicata. The most remarkable group of animals be- 

 longing to this order are the Ascidians. The cell of the 

 Polyzoan is represented in the Ascidian by a test or tunic, 

 of fleshy or membranous consistence, having two orifices, 

 within which is another envelope, distinguished as the 

 mantle. Few microscopic spectacles are more interesting 

 than the sight of the circulation along this network of 

 muslin-like fabric, and that of the ciliary movement by 

 which the circulating fluid is kept moving. In the trans- 

 parent species, such as Clavelina and Perophora, this move- 

 ment is seen to great advantage. The animals are found 

 very commonly adhering to the broad fronds offuci, or on 

 pieces of shell, near low water-mark. They thrive in 

 tanks, and multiply, both by fissuration and budding. 

 M. Milne Edwards, Professor Forbes, and others, have 

 contributed memoirs upon these curious animals, one 

 species of which is figured on the Frontispiece. 



Pteropoda. The most prominent character of this class 

 is the possession of two broad muscular fins, one on either 

 side of the neck, somewhat resembling the expanded wings 

 of a butterfly, whence Cuvier gave them the name of 

 Pteropoda, " wing-footed." In Clio, the anatomy of which 

 has been carefully investigated, there is a very curious 

 apparatus developed for seizing its prey. On each side 

 of the mouth are three fleshy warts, covered with minute 

 red specks. Under the microscope, these specks, number- 

 ing about three thousand on each tentacle, are seen to be 

 transparent cylinders, each containing in its cavity twenty 

 stalked discs, and forming so many adhesive suckers. 



The Oyster is the type of the tribe Ostracea, and is 

 Acephalus, (non-headed,) that is an animal without a distinct 

 head. The gills, or breathing apparatus, form what is 

 commonly called the beard of the oyster. The creature is 

 attached by strong muscles to its shell. The mouth of 

 the oyster is a mere opening in the body, without jaws or 

 teeth ; its food consists of nourishing substances suspended 

 in the water, and which are drawn into the shell when 

 it is open by means of cilia. Oysters attach one of their 

 valves to rocky ground, or some fixed substance, by a mu- 

 cilaginous liquid, which soon becomes as hard as the shell 

 itself. They spawn some time in May; and their growth is 



