MANUAL OF ZOOLOGY. 



nicates with the exterior by two or more contractile apertures. 

 The nervous system consists of two ganglia, situated in front 

 of the mouth, united by a commissure, and giving off filaments 

 in various directions. Pigment-spots, or rudimentary eyes, from 

 two to sixteen in number, are often present, and are always 

 placed in the prae-oral region of the body. The male and 

 female organs are united in the same individual, and the pro- 

 cess of reproduction may be either sexual, by means of true 

 ova, or non-sexual by internal gemmation or transverse fission. 



Fig. 70. Morphology of Turbellaria. i. Pint/aria torva (Muller); m Mouth ; ? 

 Nerve-ganglion; ^Eyes: ov Ovary ; t Testis ; gn Genital opening. 2. Planaria 

 lactea, showing the branched (dendrocoel) intestine. 3. Microscopic larva of Alaii- 

 rina. a marine Turbellarian. 4. Pilidium, the " pseudembryo" of a Nemertid ; 

 a The alimentary canal ; b Rudiment of the Nemertid. 



The Planarians have been divided into two sections, as 

 follows : 



Section A. RHABDOCCELA. Intestine straight, not branched. 

 Body elongated, rounded, or oval. 



Section B. DENDROCCELA. Intestine branched or arbores- 

 cent. Body flat and broad. 



SUB-ORDER II. NEMERTIDA. The Nemertida, or " Ribbon- 

 worms/' agree in most essential respects with the Planarida. 

 They are distinguished, however, by their elongated, vermi- 

 form shape, by the presence of a distinct anus, by the posses- 

 sion of a distinct perivisceral cavity, by the absence of an 

 external aperture to the water-vascular system of the adult, 

 and by the fact that the sexes, with one or two exceptions, are 

 distinct. The Nemertida further differ from the other Platyel- 

 mia in possessing a pseudohasmal system in addition to, and 

 distinct from, the water-vascular system. 



Reproduction takes place by the formation of true ova, by 

 internal gemmation, or by transverse fission. In Nemertcs, 

 however, the egg gives rise to a larva, from which the adult is 



