668 THE ANIMAL KINGDOM. 



possess a muscular sucker situated near the centre of the body : others 

 (Leptoplanidae) have a sucker between the male and female generative 

 openings. A system of dorso-ventral fibres traverses the parenchyma. 

 Its fibres branch at each end, and are attached to the basement membrane. 

 The parenchyma consists in Acoela of a soft granular protoplasmic mass 

 into which the food passes, a digestive tract not being differentiated. In 

 Rhabdocoela and Alloiocoela it consists of delicate connective tissue-fibres 

 with nuclei which form a reticulum, and connective tissue cells. The 

 coelome is formed by the spaces between these elements and the viscera. 

 The degree to which it is developed varies much, and the fluid which fills 

 it is usually colourless. On the contrary the parenchyma of Polycladida 

 is composed apparently of nucleated cells, containing vacuoles, and filling 

 up the whole space between the viscera. A coelome is consequently 

 wanting. Pigment is scattered in the parenchyma, in Rhabdocoelida in 

 special cells 1 . Convoluta paradoxa possess peculiar yellow cells, containing 

 plates of diatomin (?) ; Vortex viridis, Convoluta Schultzii, &c., cells con- 

 taining chlorophyl and starch granules. These coloured cells usually lie 

 near the integument and are perhaps symbiotic algae (see pp. 242-4). 



A nervous system does not exist in Acoela' 2 '. In other Turbellaria 

 there are a couple of ganglionic swellings usually placed anteriorly, some- 

 times, as in Polycladida, more posteriorly, in front of or above the pharynx. 

 The two ganglia are usually more or less closely united, and each of them 

 gives origin, except in Polycladida, to a nerve which runs backwards 

 ventral to the digestive system. These cords are rarely united by a 

 transverse commissure in Rhabdocoela, but in the Tricladida they possess 

 many such commissures. The Polycladida have several stout nerves 

 radiating from the ganglia, as well as others from the longitudinal cords. 

 Numerous commissures unite them one with another. The branches given 

 off by the lateral nerves ultimately form a sub-muscular plexus. In 

 addition to the tactile ectodermic cells which are chiefly grouped about the 

 anterior region of the body, or in Polycladida on the dorsal papillae and 

 tentacles, both eyes and auditory organs are found. As to the former, 

 pigment spots without a lens lie in the ectoderm of Acoela and the Micro- 

 stomidae (Rhabdocoela}. But in all other Turbellaria the eyes when present 

 lie in the parenchyma. In the Rhabdocoelida they sometimes lie in the 

 ganglia themselves. There are usually two or four in this group, and in 



1 The fore-part of the body in Derostomum^-d) Benedeni is coloured red with haemoglobin 

 (Francotte). 



2 But Convoluta Schultzii is stated by Delage (A. Z. Expt. (2), iv. 1886) to have a ganglionic 

 mass surrounding the otolith, with two other and smaller masses connected inter se and to it. Six 

 parallel longitudinal nerve-fibres extend backwards. They lie apparently on the dorsal aspect, are 

 connected by transverse commissures, and unite posteriorly in a plexus. There is a peculiar frontal 

 sensory organ. Lang mentions (p. 672, Polydaderi) an Acoelan with well developed nervous 

 system. 



