422 THE CANADIAN NATURALIST. [DeC. 



definite stages, before attaining the perfect form, and one ovum 

 may produce several adult Cyaneas. 



Another very common species on our coasts is the white or 

 colourless Jelly-fish, Aurelia Jiatidula. It has four white or 

 milky spots (the ovaries) seen conspicuously through its trans- 

 parent body, and has short marginal tentacles. 



THE TUNICATES. 



Externally these creatures are among the most uninteresting of 

 the molluscs ; their whole bodies being enclosed in a uniform sac- 

 like coat. A species of Boltenia, (B. Bolteni, Linn.) presenting 

 externally the appearance of a leathery sac, supported on a stalk, 

 is not uncommon on our coasts. (Fig. 92.) 



Boltenia Bolteni, Linn., Gulf of St. Lawi-ence— reduced. 



The sac has two apertures, and when the animal is alive, the 

 sea-water is drawn into one of these, and expelled from the other 

 by the alternate contraction and expansion of the sac. On dis- 

 secting the outer tunic, this is found to be lined with a muscular 

 sac, which is the true mantle, and by the contraction of which 

 water is expelled from the interior, while it is re-admitted by the 

 elastic expansion of the outer tunic. Within the muscular sac is 

 a delicate membraneous ciliated organ, the respiratory sac, along 

 the surface of which the water entering by the entrant aperture 

 is carried by the motion of the cilia, and the nutritive matter 

 which it contains wafted toward the mouth, which lies near the 

 bottom of the sac. The intestine doubles round and empties at 

 the excurrent aperture, toward which also the opening of the 

 ovarian ducts is directed. The creature, thus constituted, 

 remains attached at the bottom of the sea, and its actions are lim- 



