THE TURBELLARIA 



It is by no means certain that the " rod cells " of the Turbellaria 

 are the direct descendants of cnidoblasts of the ordinary Coelen- 

 terate ; more likely is it that they have a close affinity with the 



-CL 



FIG. IV. The Microscopic Structure of the Body Wall in Turbellaria. 



1. A Triclad (after Woodworth). a, epidermal cell ; &, rhabdites or rods, formed in sub- 

 epidermal cells, and passing upwards between the ciliated cells ; c, a sub-epidermal bacilliparous 

 or rod cell. In many Triclads these cells do not sink below the epidermis ; d, basement mem- 

 brane ; e, circular muscles ; /, longitudinal muscles ; g, vertical muscles ; h, nucleus of 

 parenchymal syncytium ; i, lacunae in the parenchyma. 



2. A Polyclad (combined from Lang's figures), a, epidermis ; h, rhabdite cell ; <J, "pseudo- 

 rhabdites " in an epidermal cell ; .?', parenchymal syncytium ; k, the mlclens of a muscle cell 

 {or myoblast) ; I, gland cell ; o, oblique or diagonal muscles ; d, e, f, g, i, as before. 



3. The Polyclad Anonymus (after Lang), o., b, rhabdite cells ; another in the middle is in 

 the act of discharging a rhabdite ; c, "needles" (striated rods) in their parent cell ; n, nemato- 

 cyst in its parent cell ; this is the uppermost of a tract of cells containing other nematocysts, 

 needle cells, and a sagittocyst ; such a tract constituting a u'a/cimtnisse is rare amongst 

 Polyclads, but common enough amongst Rhabdocoels, where it is formed, however, usually, 

 of rhabdites only ; S, a sagittocyst ; /, outer, and /', inner layers of longitudinal muscles ; other 

 letters as before. 



4. A Rhabdocoel (combined from Vejdovsky, etc.). b, rhabdite cell, in its primitive 

 position, as occurs in some forms; c, sub-epiilermal rhabdite cell, such as occurs in other 

 forms ; (/, an epidermal cell, in which rhabdites are commencing to be formed ; j, branched, 

 central parenchymal cell ; /, peripheral parenchymal cells ; i, intercellular lacunae ; other 

 letters as before. 



5, 6, 7. Three stages in the development of rhabdites in epidermal cells of the Polyclad 

 Thysanozoon (from Lang), n, nucleus of cell : x, refringent globules of secretion which develop 

 into the rhabdites (r). 



8. A sagittocyst from the acoelous genus Conwluta (from v. Gr.). s, the sagitta. 



9. The same, discharging its sagitta. 



10. A nematocyst from the Rhabdocoelid MicroKtoma lineare (after v. Gr.). The thread is 

 everted, but the cyst itself remains embedded in its parent cell or "cnidoblast." 



adhesive cells of Ctenophora. But the existence of true nematocysts 

 in several species does not forbid us deriving the group from a 

 more generalised Coelenterate. 



