HISTOLOGY OF THE LAND-PLANARIANS OF CEYLON. 
149 
gous in the two animals. Further, were leeches really tertiary aggregates, the fact 
would surely come out in their development, or at least some indication 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 proto vertebrae in verte- 
brates, which Mr. Spencer calls secondary aggregates. If Mr. Spencer’s hypothesis 
were correct, we should expect to find at least some Annelid developing its segments in 
the egg as a series of buds. It is not, of course, here meant to be concluded that 
Annelids are not sometimes in a condition of tertiary aggregation, as Nais certainly is 
when in a budding condition, but that ordinarily they are secondary and not tertiary 
aggregates ; and if so, then so also are Arthropoda. 
Much more information concerning the anatomy of Planarians will be required before 
it will be possible to trace the line of descent of Bipalium and Rhynchodemus , and 
determine what was the form of their aquatic ancestors. In the absence of accurate 
accounts of the structure of the American Land-Planarians, and even of the European 
Rhynchodemus terrestris, the question is very puzzling. The formation of either one of 
the two forms Bipalium or Rhynchodemus might be accounted for with comparative 
ease, from the arrangement of parts in the flat head of Bipalium. From the tree-like 
branching of the digestive tract in that region, the corresponding ramification of the 
vascular system, and general muscular arrangement, it might be imagined that Bipalium 
had come from a flattened parent of the common Planarian form, and that all the body 
except the head had become rounded and endowed with an ambulacral line. In nearly 
all points, except the eyes and the absence of branches to the oviduct, Bipalium seems 
more highly specialized than Rhynchodemus. We might imagine that Rhynchodemus 
and Bipalium had a common parent, and that when an ambulacral line was just 
beginning to be developed the two forms took different lines — Rhynchodemus losing 
all traces of the original flatness of its ancestor, and never developing any ciliated sacs 
or papillas, but cherishing a single pair of large eyes at the expense of all the rest 
which it possessed, its testes, moreover, remaining in a comparatively primitive condition. 
But then comes the difficulty about the great difference in shape in the pharynxes of 
the two forms ; and if it be suggested that, as is highly probable, several or many aquatic 
Planarians have taken to terrestrial habits, and that Bipalium has been derived from a 
form like Leptoplana , with a folded pharynx, whilst Rhynchodemus came from an 
ancestor with a tubular one, it is difficult to account for the many points of close 
resemblance between these two forms, and especially their similarity in external 
colouring, though this latter may perhaps be explained by mimicry. On the whole, it 
is evident that a close study of the anatomy of Land-Planarians cannot fail to lead to 
interesting results ; and it is hoped that this memoir may lead to further work of the 
same kind. It would be of especial value to have a good account of the anatomy of 
Geodesmus and Rhynchodemus sylvaticus. 
