214 UNSEGMENTED WORMS 



Classification. — 



Order i. Rhabdocoelida — small fresh-water and marine forms. 

 The food canal is very slightly branched, or quite straight, or 

 blocked. 

 Rhabdocoela. 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. Grafilla and Anoplodium are parasitic on 

 Gastropods. Among the Vorticidae allied to Grafilla we may 

 notice Provortex tellincB in Tellina and a related form in the 

 cockle. 

 Alloiocoela. With irregular caeca on the gut, e.g. Allostoma. 

 All marine except one from Swiss lakes {Plagiostoma 

 lemani) and Bothrioplana. 

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

 symbiotic alg«. 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) ; Bipalium 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, e.g. Temnocephala, 

 found clinging to fresh-water animals, especially Crustaceans ; 

 there is a large ventral sucker ; the epidermis is a nucleated 

 syncytium (i.e. without distinct demarcation into cells) which 

 secretes a thick cuticle, contains rhabdites, and rarely bears 

 cilia. The class seems to be intermediate between Rhab- 

 docoelid Turbellaria and Trematodes. 



Symbiotic algse. — Of all the numerous Invertebrates 

 which harbour symbiotic algae within their bodies the 

 best studied is the Acoelan Convoluta, thanks especially 

 to Keeble and Gamble. In C. roscoffensis the algae are 

 green {Zoochlorellce) ; like other green plants, they utilise 

 the energy of sunlight to build up complex organic com- 

 pounds from carbon dioxide, with evolution of oxygen. 

 Both the oxygen and the elaborated compounds (food- 

 stuffs) are valuable to the host ; for the greater part of its 

 life C roscoffensis does not feed for itself, but lives on the 



