532 THE MICROSCOPE. 



Pholas from Conferva*, which, when they get at it, grow 

 not merely outside, hut even within the lips of the 

 valves, preventing the action of the siphons. In the foot 

 there is a gelatinous spring, or style, which when taken 

 out has great elasticity, and which seems the mainspring 

 of the motion of the Pholas dactylus." 



Timicata, — The most remarkable group of animals be- 

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

 Polyzoon is represented in the Ascidianhy&test or tunic — 

 from which they derive their name — of a membraneous or 

 cartilaginous consistence, and often including calcareous 

 spicules, — having two orifices, within which is another 

 envelope, distinguished as the mantle. Few microscopic 

 spectacles are more interesting than the sight of the circu- 

 lation along this network of muslin-like fabric, and that 

 of the ciliary movement by which the circulating fluid is 

 kept moving. In the transparent species, such as Clave- 

 lina aud Perophora, this movement is seen to great advan- 

 tage. The animals are found very commonly adhering to 

 the broad fronds of fuci, or on pieces of shell, near low 

 water-mark. They thrive in tanks, and multiply both bj 

 fissuration and budding. Two species are figured in Plate 

 IX. i and k, Botryllus violaceus belonging to the family 

 Didemnians, the zooids of which are often arranged in the 

 beautiful stellate clusters seen in the plate. 1 



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, 

 numbering 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. 



(1) For information respecting the Compound Ascidians, see the admirable 

 monograph of Milne-Edwards, Art. Tunicata in the Cyclop. Anatomy and Phy- 

 siology, Huxley in Phil. Trans, for 1851, or Joum. Micros. Soc. voL iv. 1S56 ; 

 also Prof. Allman, same journal, voL vii. 1S59. 



