266 RECOED OF CURRENT RESEARCHES RELATING TO 



among the larger ones in tlie marine Planaria. As, however, the 

 brain in the terrestrial forms is not surrounded by a firm capsule, it is 

 difficult always to distinguish between the cells of the brain and 

 those of the surrounding connective tissue. When the longitudinal 

 nerves are carefully prepared, they may be seen to be made up of a 

 fine dotted substance, altogether similar to that of the cerebral ganglia, 

 together with a network of fibrils such as may be found in the trans- 

 verse section of the lateral nerve of any Nemertine. The two longi- 

 tudinal nerve-trunks of all Planarians give off a number of branches 

 to the skin and to the organs of the body ; moreover, it is quite easy 

 in Bhynchodesmus, though less easy in Geodesmus, to show that the 

 two trunks are continually united to one another by transverse com- 

 missures. In the former of these two there are two small pigment 

 goblets, which are filled with small cells, while the pigment is black ; 

 in the latter the eyes are larger, the pigment is dark brown, and the 

 cells are elongated and hexagonal ; the mode of connection of these 

 eyes with the spots has not been completely worked out. In some 

 forms it is also possible to distinguish two bright spots near the 

 eyes, which are destitute of pigment and rods, and in which the epi- 

 dermis takes on the character of small cylindrical cells with fine, 

 short cilia. To these spots a very strong nerve is given off from the 

 brain, and this swells out in such a way that we may almost say that a 

 ganglion is developed ; it is possible that these organs belong to the 

 same category as the " lateral organs " of the Nemertine worms, as do 

 also the granular aggregations found by Keferstein in the marine 

 Planaria. 



Nothing is definitely known as to the development of their nervous 

 system, but it may be presumed that it commences from an unpaired 

 rudiment ; the commissures would then be indications of this primitive 

 unity, and while in some {Bhabdocoela, Bipalium) the two longitu- 

 dinal cords finally become distinct, it is possible that the " step- 

 ladder" nervous system of higher worms will find here its explana- 

 tion. We know, indeed, that in Tomopteris the longitudinal nerve- 

 cords have a very close resemblance to what is found in the Planaria. 

 The oesophageal ring may be explained by the characters of the 

 mouth and pharynx, just as in Planaria llmuli the sucker gives rise to 

 the union of the longitudinal nerves. Among the Turbellaria, Micro- 

 stomum has an oesophageal ring ; and if the pharynx of the Planaria is, 

 as Semper thinks, homologous with the proboscis of the Nemertinea, 

 then the latter have a step-ladder nervous system. 



As to the water-vessels, the author has not been able to detect any 

 indication of any canals. This he thinks to be due to the absence of 

 proper walls and a definite epithelial investment ; but there seem to 

 be canalicular spaces in the tissue of the body, which have openings, 

 though not definite openings, to the exterior, and in which a clear 

 fluid is driven along by cilia ; the cells which develop these last are 

 not aggregated into a distinct epithelium. These suggestions afford 

 an explanation of the apparent absence of the water-vascular system 

 when we reflect that on the contraction of the animal at death the 

 lacunte in the body would disappear. 



