i8a 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 Leptoplana, the 

 "living film" found on the shore-rocks. Fresh-water forms are 

 usually small 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 I. 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. Microstoma^ 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. Graffilla and Anoplodium are parasitic on 

 Gastropods. Among the Vorticidse allied to Graffilla we may 

 notice Provortex tellina in Tellina and a related form in the 

 cockle. 



Alloioccela. With irregular caeca on the gut, e.g. Allosloma. 

 All marine except one from Swiss lakes (Plagiostoma 

 lemani) and Bothrioplana. 



Accela. Without intestine, e.g. Convoluta^ which contains green 

 cells, regarded by some as symbiotic Algse. Marine. 



Order 2. Tricladida. Elongated flat "Planarians" with 

 three main branches from the gut, e.g. Planaria and 

 Dendroccelum (fresh -water), the former sometimes dividing 

 transversely ; Polycelis nigra^ a common fresh-water form ; 

 Gunda (Procerodes) segmentata (marine), showing hints of 

 internal segmentation ; Geodesmus and Bipalium (in damp 

 earth) ; Bipahum 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 TEMNOCRPHALOIDEA 



The Temnocephalids are flattened forms, e.g. Temnocephala, 

 found clinging to fresh-water animals, especially Crustaceans : 

 there is a l?.rge ventral sucker ; the epidermis is a nucleated 



