484 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
refractive and probably calcareous. They recall exactly the bodies 
found in Cestodes, and the author suggests that the free Cestodes which 
have been described from the Lake of Geneva are nothing more than 
dead and altered examples of this Nemertine. The new worm is from 
25 to 30 mm. long. 
Development of Rhabdocoela and Triclads.* — M. P. Hallez points out 
a developmental law in these two groups, which he unites as Turbellaria 
diploblastica, while eliminating the order Dendrocoelida. The larvae of 
these animals are spherical or slightly ovoid ; and the mouth occupies one 
extremity. The provisional pharynx, which has no sheath, is terminal, 
and recalls the arrangement of the mouth in the ciliated larvae of the 
Anthozoa. If we make the mouth point downwards, we may divide the 
body into a cephalic and a caudal hemisphere. These two undergo 
unequal development ; the latter may grow more quickly than the former, 
when, as a consequence, the pharynx of the adult is more or less anterior, 
and the buccal oritice is directed forwards ; e. g. Plagiostoma rufodorsatum. 
When, in a second case, the rapidity of growth is equal for the two 
hemispheres, the pharynx of the adult is median, and the axis of the 
pharynx is perpendicular to the ventral surface ; e. g. various Mesostoma. 
Thirdly, the caudal hemisphere may grow less quickly than the cephalic, 
when the pharynx becomes situated in the second half of the body, and 
the buccal orifice is directed backward; e. g. Monotus fuscus and the 
Triclades. 
The unequal development of the two hemispheres gives rise to other 
consequences; the point of insertion of the pharynx into the intestine 
appears to be in the direction of the most rapid growth, the mouth 
remaining fixed. The conversion of the primitive radial symmetry into 
the bilateral symmetry of the adult is also a consequence of the unequal 
growth of the hemispheres. The long axis of the ovoid body is the 
antero-posterior axis of the adult, but does not correspond to the long 
diameter of the ovoid larva. 
As the pharynx becomes displaced from the anterior extremity back- 
wards the body becomes gradually flattened. While it is almost ovoid 
in species with an anterior pharynx, it becomes almost ribbon-like in 
the Monotidae and the Triclades ; while there is a complete series of 
intermediate types between these two extreme forms. The species with 
a posterior pharynx pass successively, in the course of their individual 
development, through stages in which the pharynx is anterior and then 
median. Those are the best swimmers that have their pharynx anterior, 
as the body is more ovoid in form ; those with a median pharynx crawl 
or swim indifferently, while those with a posterior pharynx only crawl. 
The triclad intestine is a consequence of the exaggeration of the flattening 
of the body. 
If we represent by M Y the amount of movement of the caudal hemi- 
sphere, and by m v that of the cephalic, we have three cases. 
(1) MV >w». The pharynx is situated in the anterior half of a 
more or less ovoid body; the animal swims; the testes and vitelline 
glands are compact. 
(2) MY = mv. The pharynx is median, body flattened ; the animal 
Comptes Rendus, cxiv. (1892) pp. 1033-5. 
