324 FRESH-WATER BIOLOGY 



larger fresh-water forms are usually inconspicuously colored, gray, 

 brown, or blackish or are entirely free from pigment. The smaller 

 forms are often brilUantly colored, yellow, orange, red, or rose; 

 and a few appear green due to the zoochlorellae or symbiotic 

 one-celled plants which live within the mesenchyma. The color 

 is more or less affected by the food contained in the intes- 

 tine. This is especially true of the non-pigmented or very trans- 

 parent forms and in many cases examination with a lens will be 

 necessary to show whether pigment is actually present or not. 



The anterior end is often modified so as to suggest the form 

 of a head, either by the presence of the various special sense 

 organs, a pair of lobes or cephalic appendages, or by a groove 

 or constriction separating it from the rest of the body. Eyes 

 may or may not be present. If present, the usual nrmiber is two, 

 though some forms have four and one genus of planarians, 

 Polycelis, is characterized by the possession of a large number 

 of eyes. Accessory eyes or pigment spots are common among 

 certain species. The normal eyes are usually bean-shaped and 

 are black in color although there are many exceptions. Acces- 

 sory eyes are usually more or less irregular in shape as well as in 

 position. 



A pair of sensory pits occurs in the anterior region in many 

 forms. These may be round, oblong, or sHt-shaped, and very 

 shallow or deeply sunken. They are connected with special 

 brain gangha, are usually provided with long cilia, and are re- 

 garded as olfactory organs. A few forms possess a statocyst 

 (otocyst) or balancing organ. It consists of a membranous sac 

 filled with a fluid in which a strongly Kght-refracting statoHth 

 (otolith) is suspended. The non-pigmented, light-refracting organs 

 found in Stenostomum posterior to the brain and coimected with 

 it by nerves are of three types. They may consist (i) of a va- 

 riable number of spherical bodies arranged in the form of a 

 convex organ, the so-called saucer-shaped or patelliform organ, 

 (2) of a vesicle which contains a strongly light-refracting lens- 

 shaped body on its wall, or (3) of a hollow capsule-like vesicle. 



The epidermis consists of a single layer of ciliated cells. The 

 ciUa are conspicuous in the rhabdocoels, which are enabled 



