go ZOOLOGY 



canal is a straight and unbranched cavity, such as exists in 

 Mesostoma. The other great division of the class, the Den- 

 drocoela, have a branched stomach, the main division of which in 

 the Polycladida is ciliated, the branches only being lined with 

 an amoeboid epithelium, which digests intracellularly. The 

 branches may anastomose, and in rare instances, Yungia and 

 Cycloporus, they open on to the exterior. 



The excretory system is absent in Acoela; in Ehabdo- 

 coela the main trunk may be single and open posteriorly, 

 or double, and then the two ducts may, as in Mesostoma, open 

 separately and near the mouth, or together at the posterior end. 

 In the Dendrocoels the main trunks open by paired apertures 

 situated on the dorsal surface. The main trunks may be 

 ciliated, the finer branches are probably always intracellular, 

 piercing the cells and not lying between them. 



The two main nerve cords are rarely connected by trans- 

 verse commissures in the Rhabdocoels, but in the Triclads this 

 is the usual arrangement. In the Polyclads there are many, 

 usually eight, nerve cords which diverge from the central cere- 

 bral ganglion (Fig. 63). Sensory cells provided with tactile 

 hairs occur in the ectoderm. The eyes are usually two or four 

 in number, but they may be more numerous, and in the Poly- 

 clads they increase by division. Auditory vesicles also occur, 

 though they are rare in the Dendrocoels; they are often single, 

 and consist of a vesicle fuU of fluid in which a calcareous 

 otolith floats. The anterior end of the body is remarkably 

 sensitive, and in some genera forms a tactile proboscis which can 

 be retracted into a shfeath ; this, together with a pair of lateral 

 ciliated grooves which lie one on each side of the brain in many 

 Ehabdocoels, affords matter for a comparison with the members 

 of the class Nemertea. 



Turbellarians, with the exception of two genera, Micro- 

 stoma and Stenostoma, are hermaphrodite, but many of them 

 are protandrous — that is to say, their male reproductive cells 

 mature before the female. Most Ehabdocoels and all Tri- 

 clads have a common genital opening for both male and female 

 ducts ; others have separate apertures, and then the male is 

 usually anterior to the female. Self-fertilisation is said to 

 occur in the summer eggs of some Ehabdocoels. 



