12 THE TURDELLARIA 



Further Remarks on the Rhabdocoelida. The Rhabdocoelid Tur- 

 bellaria contain forms which approach more nearly than do any 

 other of the orders to the ancestral Platyhelminth. The anteriorly 

 placed mouth, the comparatively simple " sucking pharynx," lead- 

 ing into a simple, straight intestine ; the symmetrical condition of 

 the excretory organs ; the " compact gonads," and the frequent 

 occurrence of "ovaries" without differentiation into germ-pro- 

 ducing and yolk-producing regions all agree with a generalised 

 flatworm. Nevertheless, the nervous system in the Rhabdocoela 

 is much more highly differentiated than in the Polycladida. Of 

 the three sub-orders included in the Rhabdocoelida it is amongst 

 the Rhabdocoela that we find these simple conditions, e.g. amongst 

 the Macrostomidae ; for the Acoela, which von Graff places at the 

 base of the Turbellarian tree, present every evidence, anatomically 

 as well as embryologically, of degeneration. The Alloiocoela, 

 through Bothrioplana, lead onwards to the Tricladida. Whilst 

 the Polycladida are descended from the Rhabdocoela along another 

 line. 



The Rhabdocoela are represented abundantly in fresh water, 

 and in the sea ; some few genera even are ectoparasitic ; the 

 Acoela are marine, as also are the Alloiocoela, with the exception 

 of Plagiostoma lemani in the deep water of Swiss lakes (Duplessis), 

 and Bothrioplana, which occurs in springs. In form they are flat 

 or cylindrical, but with flat ventral surface ; ovoid, with frequently 

 the posterior end pointed. 



The epidermis in Rhabdocoelida as in the rest of the Turbellaria 

 consists of a single layer of ciliated cells (discovered by Fabricius), 

 which are usually columnar or cubical in shape (Fig. IV.). Belong- 

 ing to this layer, though in Rhabdocoelida usually sinking below 

 it, are gland cells of two chief kinds : (a) cells producing unformed 

 secretion or " mucus " ; these are specially developed, though not 

 exclusively, on the ventral surface ; and (b) cells producing 

 " formed secretion " ; the products of these cells are either 



(A) Finely granular, block-like masses, with uneven surface, 

 pseudorhabdites, v. Gr. (schleimstabchen, Lang), (IV. 2, c), especially 

 in the Alloiocoela; (B) spindle-shaped, homogeneous, refringent 

 rods, with smooth surface, "rhabdites" (discovered in Rhabdo- 

 coeles by 0. Schmidt ; and in Pclyclads by Quatrefages), (IV. 7) ; 

 (C) or each cell may produce a capsule or cyst (sagittocyst), (IV. 

 8, 9), in which is contained a single spindle-shaped needle (these 

 are rare in Rhabdocoela) ; or finally (D), in rare instances nemato- 

 cysts (Leuckart, 1848), (IV. 10), (Microstoma and Stenostoma). The 

 occurrence of nematocysts, quite similar to those of Hydra, in 

 several Turbellaria, and in certain Nudibranch Molluscs seems 

 to indicate the very close relationship of these two groups with 

 the Coelenterata. 



