174 



MANUAL OF ZOOLOGY. 



nicates with the exterior by two or more contractile a-pertures. 

 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 al- 

 ways 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. 50. — Morphology of Turbellaria. I. Platiaria torva (Muller); m Mouth; g 

 Nerve-ganglion; tf Eyes; iW Ovary; ^Testis; ^« Genital opening, a. Planaria 

 ■ lactea, showing the branched (dendrocoil) intestine. 3. Microscopic larva of Alaur- 

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

 a The alimentary canal; b Rudiment of the Nemertid. 



The Plflnarians 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 arborescent. 

 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 

 distirict. The Nemertida further differ from the other Platyel- 

 tnia in possessing a pseudohsemal 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 Nemertes, 

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



