TUEBELLAEIA. 309 



CLASS I. PLATYHELMINTHES 



Vermes with aflat, more or less elongated body, with cerebral gan- 

 glion. They are often provided with suckers and hooks, and are usually 

 hermaphrodite. 



The series of forms included under this class are mostly Entozoa, 

 or else live in the mud and beneath stones in the water. In their 

 organization they occupy the lowest place among the worms. Their 

 body is more or less flattened, and is either unsegmented or is divided 

 by transverse constrictions into a number of successive divisions, 

 which, although forming parts of one animal, yet have a strong 

 tendency towards individualisation, and frequently attain to separa- 

 tion and lead an independent life. These segments are products of 

 growth in the direction of the long axis of the body, and stand in a 

 special relation to reproduction. They are by no means to be con- 

 sidered as necessarily indicating a high grade of organization, as does 

 the segmentation of the Annelida. The alimentary canal may be 

 altogether wanting (Cestoda), or, if present, may be without an anus 

 (Trematoda, Turbellaria}J The nervous system is usually composed of a 

 double ganglion above the oesophagus, giving off small nerves anteriorly 

 and laterally, and two stems backwards. In many Platyhelminthes 

 ^eye-spots occur, either with or without refractive bodies, and 

 more rarely there is an auditory vesicle. Blood-vessels and organs 

 of respiration are found only in the Nemertinea. The excretory 

 (water vascular) system is everywhere developed. With the excep- 

 tion of the Microstomidce and Nemertinea, hermaphroditism is the 

 rule. The female generative glands consist of distinct yolk-glands 

 and ovaries. The development very frequently takes place by a very 

 complicated process of metamorphosis connected with alternation of 

 generations. 



Order 1. 



Free living Platyhelminthes with oval or leaf-shaped body, with 

 soft skin covered with cilia. They possess a mouth and aproctous 



* Duges, " Recherches sur Porganisation et les moeurs de Planaires," Ann. 

 des Sc. Nat., Ser. I., Tom XV. A. S. Oerstedt, " Entwurf einer systcinatischen 

 Eintheilung und speciellen Beschreibung der Plattwurmern," Copenhagen, 

 1844. De Quatrefages, " Memoire sur quelques Planariees marines," Ann. des 

 Sc. Nat., 1845. M. Schultze, " Beitrage zur Naturgeschi elite der Turbellarien," 

 Glreifswald, 1851. L. Graff, " Zur Kenntniss der Turbellarien," ZeiUchnft fur 

 Wig*. Zool., Tom XXIV. L. Graff, "Neue Mittheilungen iiber Turbellarien." 

 Zeitscli. f. n-iss. Zool., xxv., 1875. P. Hallez, " Contributions 4 1'histoire 

 uatnrelle des Turbellarics," Lille, 1879. 



