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.
1 See Silliman, Z. W. Z. xli. 1885, pp. 59-63. Mesostoma viviparum and Planaria gracilis from the fresh waters; Vortex Warrenii, marine; all N. American.
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 Cienophora; 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 Cienophora, 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 Cienophora the muscle-cells are either ectodermal, or cells lodged in the supporting jelly into which they are said by most authorities to wander from the ectoderm.
MetschnikofT has quite recently described (Z. W. Z. xlii. 1885) the formation 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 compact; 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) Tricladida: body elongate; intestine with three main branches; a compact germarium, follicular vitellarium; an atrium genitale. Gunda marine; Pla-naria, Dendrocoelum, Polycelis, freshwater; and various Land Planarians, e. g. Rhynchodemus, Geodesmus, Bipalium.
(b) Polycladida: body leaf-like, broad; a central stomach; testes and ovaries follicular; with or without a ventral sucker (Cotylea, Acoiylea); marine.
'Planarians,' von Graff, Encyclopaedia Britannica (ed. ix.) xix. 1885.
Rhabdocoelida, von Graff, Monographic der Turbellarien, Leipzig, 1882; cf. Moseley, Nature, xxvii. Convoluta, Yves Delage, Etudes, etc, A. Z. Expt. (2), iv. 1886. Mesostomum, Jaworoski, Z. A. ix. 1886. Derostomum Benedeni, Francotte, Bull. Ac. Roy. Belg. (3), vi. 1883. Graffilla, Bohmig, Z. W. Z. xliii. 1886. Excretory system of Microstoma, Zacharias, Z. A. viii. 1885. Development of Acoela, Pereyaslawzew and Repiachoff, Z. A. viii. 1885.
Tricladida: Gunda segmentate Lang, Mitth. Zool. Stat. Naples, iii. 1882; of freshwaters, Iijima, Z. W. Z. xl. 1884; of N. American freshivaters, Silliman, Z. W. Z.
xli. 1885. Land Planarians, Moseley, Ph. Tr. 164, 1874; Id. Q. J. M. xvii. 1877;
Kennel, Arb. Zool. Zoot. Inst. Wurzburg, v. 1879. Transverse fission, Zacharias, Z. W. Z. xliii. 1886, p. 271. Nervous system, Lang, op. cit. (supra).
Excretory system of Rhabdocoela and Dendrocoelida (Triclads), Francotte, Archives de Biologie, ii. 1881.
Polycladida, Lang, Fauna und Flora des Golfes von Neapel, xi. Leipzig, 1884.
For Rhodope Veranii = Sidonia elegans, which appears to be a Turbellarian, but was formerly referred to the non-palliate Gastropoda, see von Graff, M. J. viii. 1882; Bergh, Z. A. v. 1882.