On the Anatomy of the Land-PJanarians of Ceylon. 313 



by so many authors, does not exist. There is no doubt that 

 Schmarda mistook the ovaries and testes for ganglia. The real 

 nervous system is ill-defined, but appears to consist of a network 

 of fibres without ganglion-cells, which lies within the primitive 

 vascular canals. In Leptoplana tremellaris the structure of the 

 ganglionic masses is remarkably complex in the arrangement of 

 the fibres ; and well-defined ganglion-cells of various sizes are pre- 

 sent and have a definite arrangement. 



Numerous eye-spots are present in Bipalium, most of them 

 being grouped in certain regions in the head, but some few being 

 found all over the upper surface of the body, even down to the 

 tail. The eye-spots appear to be formed by modification of single 

 cells. In Rhynchodemus two eyes only are present. All gradations 

 would appear to exist, between the simple unicellular eye-spot 

 of Bipalium and the more complex eye of Leptoplana or Oeodesmus, 

 where the lens is split up into a series of rod-like bodies, forming 

 apparently a stage towards the compound eyes of Articulata. 

 It is quite probable that these compound eyes have arisen by such 

 a spHtting-up into separate elements of a single eye, and not by 

 fusion of a group of unicellular eyes, such as those of Bipalium. 

 A pecuhar papillary band runs along the lower portion of the 

 margin of the head of Bipalium. The delicate papillae are in the 

 foru) of half cylinders, ranged vertically side by side. Between 

 the upper extremities of the papillse are the apertures of peculiar 

 ciliated sacs. The papillse, from the mode in which the animal 

 makes use of them, are probably endowed with a special sense- 

 function. The sacs may have a similar office, or they may be in 

 connexion with the primitive vascular system, and have an excre- 

 tory function ; they may further be homologous with the ciliated 

 tubes in Nemertines. 



In considering the general anatomy of Bipalium, it is im- 

 possible to help being struck by the many points of resemblance 

 between this animal and a leech. Mr. Herbert Spencer has, in 

 his ' Principles of Biology,' placed a gulf between Plauarians and 

 Leeches by denoting the former as secondary, the latter as tertiary 

 aggregates, so called because consisting of a series of secondary 

 aggregates formed one behind the other by a process of budding. 

 It is obvious, however, that a single leech is directly comparable to 

 a single Bipalium. The successive pairs of testes, the position 

 of the intromittent generative organs, the septa of the digestive 

 tract, and, most of all, the pair of posterior caeca are evidently 

 homologous in the two animals. Further, were leeches really 

 tertiary aggregates, the fact would surely come out in their 

 development, or at least some iudication of the mode of their 

 genesis would survive in the development of some annelid. Such, 

 however, is not the case. The young worm or leech is at first 

 unsegmented, like a Planarian ; and the traces of segmentation 

 appear subsequently in it, just as do the protovertebrae in verte- 

 brates which Mr. Spencer calls secondary aggregates. If Mr. 

 Spencer's hypothesis were correct, we should expect to find at least 



