TURBELLARIA. 673 



from the stomach in Coeloplana, and pass aborally but end blindly. A right and 

 left opening near the plane of the tentacles is found in the same animal, but what 

 they lead to is uncertain. Both genera appear not to have, unlike both Polyclads 

 and Ctenophores, anything homologous with the ectodermal pharyngeal pouch of 

 the former or stomach of the latter. 



It cannot be denied that there is a general resemblance in the symmetry and 

 in the position of the chief planes of the body in the Polycladida and Ctenophora ; 

 in the existence of an oral and aboral pole in some Polyclads and all Ctenophores ; 

 both groups are hermaphrodite, and the genital organs are in relation to the 

 digestive system, but there are genital ducts and a coitus in Polyclads, which 

 do not exist in Ctenophores. Certain resemblances are to be found in the 

 mode of segmentation of the ovum, and the persistence of the Gastrula-mouth. 

 But the nervous system of Polyclads can hardly be compared with that of 

 Ctenophora, i. e. with the sensory aboral plate and ctenophoral rows of cilia and 

 sub-ectodermic plexus ; there is no trace of anything which can pass for an 

 excretory system in Ctenophora ; the Polyclads possess a true mesoblast, whereas 

 in Ctenophora the muscle-cells are either ectodermal, or cells lodged in the sup- 

 porting jelly into which they are said by most authorities to wander from the 

 ectoderm. Metschnikoff has quite recently described (Z. W. Z. xlii. 1885) the for- 

 mation in some Ctenophores of a small mass of cells from the primitive endoderm 

 cells close to the Gastrula-mouth, which mass is carried inwards through the central 

 axis to the aboral pole, where it gives rise, so he says, to the musculature of the 

 tentacles and to the wandering cells of the jelly. It is doubtful whether the cells in 

 question can be strictly considered as mesoblast. They form a small aboral group, 

 but extend no further round the body, and the mode in which they shift their place 

 is at least remarkable. For a full discussion of the relationship of Polycladida to 

 Ctenophora, see Lang, Polycladen, Fauna des Golfes von Neapel, xi. Monograph, 

 pp. 645-667. He regards the Polyclads as the primitive group of Turbellaria. 



The Turbellaria are classified as follows 



1. Rhabdocoelida : small in size, body cylindrical or depressed. 



(a) Acoela : no distinct intestine or excretory apparatus ; testes follicular ; 

 two ovaries ; an otocyst ; all marine. 



(b) Rhabdocoela : a straight intestine and complicated pharynx ; testes com- 

 pact ; an ovary, or a germarium and vitellarium ; otocysts rare ; freshwater, marine, 

 or in damp earth (Prorhynchus sphyrocephalus ; Pr. stagnalis sometimes). 



(c) Alloiocoela : intestine lobed or irregular ; a pharynx ; testes follicular ; 

 otocysts in the Monotidae : marine except Plagiostoma Lemani from the deep water 

 of Swiss lakes. 



2. Dendrocoelida : large ; intestine branched ; testes follicular ; otocysts very 

 rare. 



(a) Tridadida : body elongate ; intestine with three main branches ; a com- 

 pact germarium, follicular vitellarium; an atrium genitale. Gunda marine; Pla- 

 naria, Dendrocoelum, Polycelis, freshwater ; and various Land Planarians, e. g. 

 Rhynchodemus, Geodesmus, Bipalium. 



(V) Polycladida : body leaf-like, broad ; a central stomach ; testes and ovaries 

 follicular ; with or without a ventral sucker ( Cotylea, Acotylea) ; marine. 



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