90 RECOBD OF CURRENT RESEARCHES RELATING TO 



Classification and Phylogeny of the Turbellaria-* — Paul Hallez 



gives a sketch of the results to which he has been led by his studies. 

 After noticing the various views which have been held by different natu- 

 ralists as to the affinities of this lowly group, he comes to a consider- 

 ation of the classification suggested by Ehrenberg, who proposed to 

 divide the Turbellaria into the Dendroccela and Rhabdocoela, according 

 as the intestine was or was not branched. Speaking generally, this 

 arrangement has held its own up to the present day ; but evidence has 

 been gradually accumulating to show that the character chosen by 

 Ehrenberg is not the most important or valuable; for example, the 

 Planaria of the Lake of Geneva, discovered by Du Plessis, has 

 been shown by Graff to be a Dendrocoelous form with a straight 

 intestine. 



When, however, we consider by what characters the two groups of 

 Turbellaria are to be distinguished from one another, we find that 

 there is a series of forms which appears to present a mixture of the 

 characters of the two suborders ; such are the genera Opistomum, 

 Monocelis, and Turhella. Studying the more distinct forms, Hallez 

 finds that there is a marked difference in the extent to which the 

 reticulum of connective tissue is developed ; in the Dendroccela it 

 occupies nearly the whole of the body-cavity, while in the Rhabrlocoela 

 it is much more feebly developed. This difference, as well as the 

 others on which our author insists, will be best displayed by a 

 table : — 



Rhdbdocoela. Dendroccela. 



1. Eeticulum not well developed. Reticulum nearly obliterates the 



whole of the body-cavity. 



2. Pharynx dolioliforin (cask- Pharynx tubuliform. 



shaped). 



3. Water- vessels present. No water-vessels. 



4. Ovaries and testicles: ordi- Ovaries and testicles: more than 



narily one pair. one pair. 



5. Body more or less cylindrical. Body more or less flattened. 



Embryological investigations seem to prove that the rhabdocoelous 

 is more ancient than the dendrocoelous form, but it still remains to 

 be shown what members of the former group most resemble the 

 ancestor of the Dendroccela ; led by the characters of the mouth, the 

 absence of any proper wall to the intestine, and the multiple character 

 of the generative organs, Hallez is of opinion that the now existing 

 Convoluta and Nadina stand nearest to the stem-form. The Nemer- 

 tinea appear to pass into the Rhabdocoela through Stenostomum and 

 DinopMlus, the latter of which has an anus and a proboscis as well 

 as the lateral ciliated pits, in which points it resembles the Nemer- 

 tinea (Rhynchocoela) and differs from the rest of the Turbellaria ; 

 the Microstomese are looked upon as being the simplest of the 

 group, and as consequently the most closely allied to the Dicyemida 

 and Gasterotricha. We add the phylogenetic table by which the 

 author expresses his views as to the affinities of the different forms, 



* ' Rev. Interuat. Sci.,' iv. (1879) p. 466. 



