TURBELLARIA. 667 



tory organs wanting. Hermaphrodites with rare exceptions and the ovary 

 commonly divided into a germarium and vitellarium. 



The small Turbellaria are mostly cylindrical, or the ventral surface is 

 flat, the dorsal convex : the larger Turbellaria are usually broad and thin, 

 with the exception of those inhabiting damp earth which are long, narrow, 

 and thick bodied. The anterior extremity is usually wider than the 

 posterior, but it may be narrow and pointed, and partially or wholly 

 retractile, forming the proboscis of the Rhabdocoele Proboscida and some 

 few genera. It is occasionally provided with tentacles as in the Rhabdo- 

 coele genus Vorticeros and in many Polycladida. These tentacles are 

 either nuchal, and then solid, contractile, sometimes retractile into pits and 

 placed one on either side the nerve ganglia (Planoceridae\ or marginal and 

 anterior, and then either lobes (P seudoceridae] or pointed processes (Eury- 

 leptidae) into which branches of the intestine pass. The ectoderm consists 

 of a single layer of ciliated cells sometimes covered by a delicate cuticle. 

 Pigment rarely occurs in them, but the majority contain rhabdites clear 

 homogeneous smooth rods pointed at each end and formed probably as a 

 secretion either in the superficial cells themselves (Polycladida], or also in 

 cells of the ectoderm imbedded in the parenchyma (most Tricladida, all 

 Rhabdocoelida). Pseudorhabdites occur in Alloiocoela : they are rod-like, 

 but granular and with an uneven surface. True nematocysts are rarely 

 found, e.g. in Microstoma lineare^. Some of the ectoderm cells are 

 glandular, but unicellular glands occur also in the parenchyma with ducts 

 leading up to the surface. Adhesive cells with processes and a sticky 

 secretion are commonly found on the ventral aspect at the posterior end 

 of the body, but not in the adult Polycladida (?), though sometimes present 

 in the larva. Tactile cells with bundles of immobile but bendable hairs 

 are met with in all Polycladida and in some other Turbellaria. Beneath 

 the ectoderm is a basement membrane, the thickness of which varies much. 

 It is stout and contains richly branched cells in Polycladida. The muscu- 

 lature of the body forms a continuous investment. In the Rhabdocoelida it 

 consists of an outer circular, and inner longitudinal layer of fibres which 

 are reversed in position in Microstoma lineare, and in many instances have 

 interposed between them a diagonal layer. The layers are more compli- 

 cated in Polycladida, but consist generally speaking of an outer and inner 

 longitudinal layer, the latter absent on the dorsal surface, inclosing between 

 them a layer of circular, and two layers of diagonal, fibres. The layers 

 vary in Tricladida, but the outer layer is circular. Many Polycladida 



1 Still more rare are the structures known as sagittocysts, i. e. capsules similar to those of 

 nematocysts, but inclosing a needle-like rod, which is expelled on irritation of the animal. It is 

 very rare to find a Rhabdocoele without any of the structures named, e. g. the parasitic Graffilla, 

 or Prorhynchus stagnalis. There is some doubt as to whether the rhabdites are expelled, as is 

 usually supposed. They may serve as a skeleton. See lijima, Z. W. Z. xl. pp. 372-74. 



