30 PLATYHELMINTHES TURBELLARIA chap. 



dites (Fig. 12) arise, apparently as so many condensed secretions. 

 From the ectoderm the brain arises as two pairs of ingrowths, 

 which fuse together, and from these the peripheral nervous system 

 grows out. Three pigmented ectoderm cells give rise, by 

 division, to the eyes an unpaired cell (Fig. 12, e) to the cere- 

 bral group of eyes, and the other two to the marginal and 

 tentacular groups. The copulatory organs apparently arise to a 

 large extent as ingrowths from the ectoderm, from which the 

 accessory glands (prostates, shell-glands) are also formed. The 

 endoderm forms the lining of the main-gut and its branches. 

 The pharynx is developed as in Fig. 13, which shows that the 

 " mouth " of the young larva (C) does not correspond exactly with 

 that of the adult (D). The salivary glands arise from ectoderm 

 cells, which sink deeply into the parenchyma. The reproductive 

 organs (ovaries and testes) possibly arise by proliferation from 

 the gut-cells (Lang, v. Graff). The change from the larva to the 

 adult is gradual, the ciliary band being absorbed and the creep- 

 ing mode of life adopted. 



Turbellaria. II. Tricladida. 



The Triclads are most conveniently divided into three groups 1 ; 

 (i.) Paludicola, the Planarians of ponds and streams ; (ii.) the 

 Maricola, the Triclads of the sea; and (iii.) Terricola or Land 

 Planarians. From the Polyclads they differ in their mode of 

 occurrence ; in the elongated form of their body and almost 

 constant, mid -ventral position of the mouth ; in possessing a 

 single external genital pore (Monogopora) ; and in the production 

 of a few, large, hard-shelled eggs provided with food-yolk. 



Occurrence of the Paludicola. The Planarians of our ponds 

 and streams are the most familiar and accessible Turbellaria. 

 Their elongated, flattened bodies, and gliding movements, render 

 them conspicuous objects on the under surface of stones and on 

 the leaves of aquatic plants, where they live gregariously. The 

 variable Polycelis nigra (Fig. 14, H) is very abundant in stagnant 

 water and slowly-moving streams, whereas its ally, P. cormda 

 (Fig. 14, G), distinguished by a pair of tentacles, is more local. 

 Planaria (Dendrocoelum) lactea (A), P. polychroa (I), P. torva, 

 and P. punctata are not infrequently found together, but the last 



1 Hallez, Revue Biologique du Norcl de la France, torn. ii. 1889-90. 



