26 THE TURBELLARIA 



or extended to form a crescentic or hammer-shaped plate (Eipalium), 

 and may even bear suckers (Dicotylus, Cotyloplana). 



The rod cells sink into the parenchyma, and are connected with 

 the epidermis by "rod tracts " (Fig. IV. 1). Peculiar Y-shaped 

 rods have been recorded in Placocephalus (Shipley). In a few 

 genera rhabdites are absent (Bdelluridae). 



The parenchyma appears similar to that in Rhabdocoela. 



The post- central position of the mouth, combined with the 

 great size of the tubular pharynx, appears to have led to the 

 modification of the originally central gut, so as to form three 

 lobes one anterior lying above the brain and a posterior pair, 

 which lie at the sides of the pharyngeal pouch, and usually ex- 

 tend nearly to the hinder end of the body (Figs. XI., XII.) ; they 

 not unfrequently unite posteriorly (cf. Botlirioplami). The lateral 

 caeca may branch in the lower forms, but in the Terricola and 

 Maricola become larger and more regular in their arrangement ; 

 but they do not appear to be so strictly metameric with nerves and 

 gonads, as Lang (41) believed for Gunda segmentata and terrestrial 

 forms (Wheeler and Dendy). 



The nervous system retains to a greater extent than in the 

 Rhabdocoelida a primitive condition, in that the brain gives 

 origin to a number of nerve strands, which form a subdermal 

 network all over the body; but a ventral pair of longitudinal 

 nerves are much more prominent than the others, arid connected 

 by transverse commissures in a fairly regular way, especially 

 in Gunda segmentata. But ganglion cells remain at the origin 

 of the nerves and transverse commissures. The sense organs 

 may be (a) ciliated pits at the side of the head, receiving 

 nerves from the brain ; these are especially well developed, and 

 abundant in Bipalium and Geoplana, where they lie in a lateral 

 groove (Fig. X. 3) ; the former genus too has retractile papillae 

 round the margin of the head (Moseley). (b) Eyes are either 

 confined to the anterior end, or in many land Planarians along 

 the entire margin. Dendy has suggested that the unicellular eyes 

 (Fig. X. 2) of Geoplana, etc., are derived from modified rod cells ; 

 multicellular eyes occur in other terrestrial genera. 



The excretory system (Fig. VII. 6) presents two laterally placed 

 main canals which give off numerous special branches, opening 

 dorsally on to the external surface. In Gunda there are two main 

 lateral canals on each side connected by short canals, and the 

 external pores exhibit the same repetition noticeable in other 

 systems of organs. The main trunks are sparsely ciliated. Lang 

 discovered in Gunda certain vacuolated cells with flames amongst 

 the intestinal epithelial cells, and therefore suggests that the flame 

 cells generally have a hypoblastic origin. It is only within recent 

 years that the excretory system of Triclada has been described 



