The Museum. 
Vol. I. PHILADELPHIA, AUGUST, 1885. No. 4. 
* PLANARIANS. 
BY PROF. JOSEPH LEIDY. 
In the extensive class of vermes or worms, is an interesting group constituting 
the family of Planarians. Essentially aquatic, they are mostly marine, though many 
live in fresh water, and others are terrestrial. They resemble leeches in shape, 
and are often mistaken for such, though they possess an entirely different structure. 
They are of soft consistence, easily decomposed, and are slimy or covered with 
mucus. Ordinarily they creep or glide about by a broad, sole-like surface, extending 
the entire length of the body, in the manner of snails, or they may swim on the surface 
of water, upside down, likewise in the manner of the latter. In motion the Planarians 
may elongate and shorten the body, like leeches, but they have no distinctly annulated 
structure, as in the latter. The microscope exhibits their surface clothed with vibrating 
cilia; and imbedded in the skin, cells, containing minute rods, which are shot forth as 
threads when the animal is irritated. These cells, apparently, in their nature, accord 
with the nettling cells of the hydra and other ccelenterates. Though the Planarians 
conform to the usual bilateral symmetry 0/ shape of the body in most classes of 
animals, their digestive apparatus seems rather to approximate the condition of that 
of coelenterates. The mouth is situated at or behind the centre of the under surface 
of the body and communicates with an interior cavity directed forward. The cavity 
contains a proboscis, commonly a simple, free, cylindrical tube, which extends forward 
and joins the stomach, just as the proboscis of a medusa suspended within its bell 
opens into the stomach at the bottom of the latter. In the act of feeding the probos¬ 
cis is protruded its entire length from the mouth and projected in all directions. The 
stomach is of remarkable character and is spread throughout the solid structure of the 
body in three main branches, of which one is directed forward to the head, and the 
others, on each side, backward to the tail. As in the medusae it does not occupy a distinct 
body cavity orccelum, and has no other outlet than the proboscis. The main branches 
of the stomach divide into numerous lateral pouches, giving the entire organ a sort 
of vascular arrangement, distinguished as dendritic, or tree-like. The intervals of 
the stomach are partially occupied by a bisexual generative apparatus, which opens 
externally behind the position of the mouth. The anterior extremity of the body of 
