V] TURBELLARIA 99 



The genital organs are simple ; the ovary is not divided into 

 vitellarium and germarium and no yolk-cells are produced. It has 

 been suggested that the Acoela are the most primitive of all the 

 phylum, and that they have been directly derived from large 

 multinucleate Protozoa, but their development makes it possible 

 that their peculiarities are due to degeneracy and to their associa- 

 tion with Algae. Keeble has shown that these are sooner or later 

 digested by the host. 



The ALLOIOCOELA have a parenchyma and a hollow alimentary 

 canal which has slightly developed lateral lobes. The testes are 

 represented by scattered masses of cells without distinct ducts; 

 and the spermatozoa apparently find their way to the main vasa 

 deferentia by passing through the interstices of the parenchyma. 

 The germaria are two in number and have long ducts opening into 

 the genital atrium distinct from those of the vitellaria. 



The RHABDOGOELA are the most highly developed forms of the 

 Rhabdocoelida ; their alimentary canal is cylindrical and surrounded 

 by a mass of extremely watery parenchyma which simulates a body 

 cavity. The arrangement of the genital organs has been described 

 above. 



The first division of the Dendrocoelida, the TRIGLADA (Gr. 

 rpt-, triple, KAaSo?, a branch), derive their name from the circum- 

 stance that there are three main branches of the alimentary canal, 

 one in the middle line running forward from the inner end of the 

 pharynx, and one running backwards at each side of the pharynx. 

 There are a pair of germaria formed from the most anterior branches 

 of the great lobed ovaries ; they discharge into the same ducts as 

 the vitellaria. The uterus is an unpaired sac. The group includes 

 marine, fresh-water and terrestrial forms. Planar ia (Dendrocoelum) 

 is a common form in the streams of both Britain and Canada. 



The POLYGLADA are a marine group and are thought by some 

 authorities to be the most primitive of all Turbellaria. Their 

 name is suggested by the fact that the alimentary canal consists of 

 many branches radiating from a central stomach into which the 

 large pharynx opens. The ovary is a lobed organ not divided into 

 vitellarium and germarium. The eggs are laid in capsules which 

 contain only one or two ; these capsules are arranged in plate- 

 like masses bound together by slime. They develop into free- 

 swimming young known as Miiller's larvae. These are little oval 

 organisms provided with a ciliated band drawn out into eight longi- 

 tudinal loops, and on these the cilia are arranged in transverse rows 



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