182 UNSEGMENTED “WORMS.” 
The Turbellarian worms form an exceedingly interesting group; they 
are often beautiful, and the ciliated ectoderm and well-developed 
muscles enable them to move with singular grace. Although the 
bilateral symmetry and the distinction of anterior and posterior ends is 
quite marked, the ‘‘ mouth” or single opening of the food canal is often 
near the middle of the ventral surface. The anterior region is usually 
furnished with tactile processes. The shape of the body in the aquatic 
forms ‘is flattened and leaf-like, as in the delicate Lef/oflana, the 
“living film” found on the shore-rocks. Fresh-water forms are 
usually smal] and often minute, but those living in the sea may attain 
a length of six inches, though most are small. Land Planarians are 
elongated and more worm-like in shape; they may measure a foot or 
more in length, and are most abundant in tropical countries. Some, 
like Planaria, have so much regenerative capacity that half a dozen or 
more may be produced by cutting one into pieces. 
Classification.— 
Order 1. Rhabdoccelida—small fresh-water and marine forms. 
The food canal is very slightly branched, or quite straight, or 
blocked. 
Rhabdoccela. With straight intestine, e.g. Azcrostoma, a fresh- 
water genus. It is first male and then female (protandrous 
hermaphrodite) ; it forms temporarily united asexual chains, 
sometimes of sixteen individuals, suggesting the origin of a 
segmented type. Grafilla and Anoplodium are parasitic on 
Gastropods. Among the Vorticide allied to Graffi/la we may 
notice Provortex teliing in Tellina and a related form in the 
cockle. 
Alloioccela. With irregular cacca on the gut, e.g. Alostoma. 
All marine except one from Swiss lakes (Plagiostoma 
lemanz) and Bothrioplana. 
Accela. Without intestine, ¢.2. Covoluta, which contains green 
cells, regarded by some as symbiotic Algae. Marine. 
Order 2. Tricladida. Elongated flat ‘‘ Planarians” with 
three main branches from the gut, eg. Planaria and 
Dendrocelum (fresh-water), the former sometimes dividing 
transversely ; Polycelts nigra, a common fresh-water form ; 
Gunda (Procerodes) segmentata (marine), showing hints of 
internal segmentation ; Geodesmus and Bipalium (in damp 
earth); Bzpalium kewense is an import often found in 
Britain. 
Order 3. Polycladida. Large leaf-like marine ‘‘ Planarians,” 
with numerous intestinal branches diverging from a central 
stomach, e.g. Leptoplana (not uncommon on the seashore), 
Thysanozoon, 
Class TEMNOCEPHALOIDEA 
The Temnocephalids are flattened forms, eg. Temmnocephala, 
found clinging to fresh-water animals, especially Crustaceans ; 
there is a lerge ventral sucker ; the epidermis is a nucleated 
