314 Royal Society. 



some xliiuelitl 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 aggrega- 

 tion, 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 Plauarians 

 will be required before it will be possible to trace the line of descent 

 of Blpalium and Rhijtwhodemus, 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 Rh)/nchodemus terrestris, the question is 

 very puzzling. The formation of either one of the two forms 

 Bipalium or Rliynchodemus might be accounted for with com- 

 parative 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 R/u/ncJiorJemus. We 

 might imagine that Rhi/nchodemus and Bij>alium 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 papillfP, 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 con- 

 dition. But then comes the difficulty about the great difference 

 in shape in the pharvnxes of the two forms ; and if it be suggested 

 that, as is highly probable, several or many aquatic Planarian s have 

 taken to terrestrial habits, and that Bipalium has been derived 

 from a form like Leptoplana, with a folded pharpix, whilst 

 Rhynchodemns came from an ancestor \^"ith a tubular one, it is 

 difficult to account for the many points of close resemblance 

 between these two forms, and especially their similarity in ex- 

 ternal colouring, though this latter may perhaps be explained ])y 

 mimicry. On the whole, it is evident that a close study of the 

 anatomy of Land-Planarians caimot 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 Rhytu-hodevms 

 sylvaticu^. 



