i84 THE POPULAR SCIENCE MONTHLY. 



base, which springs from a creeping gelatinous thread. The mantle 

 is transparent as crystal, and through it you may see, as if suspended 

 in the cavity of the body, what seems the frilled edge of a ribbon of 

 snow-white lace. This is Clavelina's lung. A little sac, seen through 

 the transparent mantle and body walls, contracting and expanding 

 with a slow and measured beat, is Clavelina's heart. 



Another cloaked mollusk is Cynthia. It adheres to rocks or peb- 

 bles under a few fathoms of ocean, and has something of the form and 

 color of a blood-peach. It is known to Avatermen under the name of 

 *' sea-peach." Its mantle is tough and leathery.* 



Another and a more interesting member of the cloaked family is 

 the Salpa. In the structui'e of the heart it marks' an advance on Clave- 

 lina. Instead of a single pulsating sac, we find an auricle and a ven- 

 tricle, veins and arteries. But, Nature having advanced from a single 

 to a double heart, it would seem that she did not vet know how to vise 

 the improvement. In the Salpa we find the heart incessantly chang- 

 ing its auricle into a ventricle, its ventricle into an auricle, veins into 

 arteries, arteries into veins. 



The Salpai swim freely in the open sea and occur singly, or united 

 in long chains or rings. They are phosphorescent, and a chain of 

 united Salpae appears like a writhing, fiery serpent gliding over the 

 waves. The Pyi-osomes, which are free Salpae, congregate in vast 

 shoals, and in their phosphorescence glare like pillars of fire, green, 

 tmearthly, elfish. 



Let the edges of the mantle unite along part of their surface, but 

 remain open at the ends. The animal now will not be comjjletely tu- 

 nicated. It will be inclosed in a kind of funnel. If, now, such a man- 

 tle be drawn out into a siphon to conduct a current of water to the 

 gills, it would be of use to the animal in aiding respiration. The 

 edges of the mantle having united in this way, a siphon-bearing mol- 

 lusk, like the cockle or solen, would be simply a question of time. 

 Natural Selection would bring it about. 



Let the edges of the mantle not unite at all, we shall have a mol- 

 lusk something like the oyster. 



Remove the shell, and an oyster lies before you in irregular, rag- 

 ged outline. An opening at the sharper end, which lies near the beak 

 of the shell, is the mouth. Around the mouth are four leaf-like bodies, 

 which hang in pairs. The heart is an advance on that of Salpa, n-ot 

 in structure but in behavior. It has settled down into regular work, 

 the auricle always an auricle, and the ventricle always a ventricle. 

 The liver is a decided advance on that of Eolis, although not yet a 

 well-defined gland. The mantle is a fringed, veil-like membrane, 



' It is known that the mantle of many tunicate moUusks is non-azotized matter. Azote 

 is another name for nitrogen, and in various proportions it is found in animal tissues. 

 This is a distinguishing feature between animal (azotized) and vegetal (non-azotized), 

 matter. Chemically the plant meets the animal on the back of a tunicate mollusk. 



