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 





._.-- - 



*" ~ 



' 



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



1. A Triclad (after Woodworth). a, epidermal cell; b, 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 h'gares). a, epidermis ; b, rhabdite cell ; c, "pseudo- 

 rhabdites " in an epidermal cell ; j, parenchymal syncytium ; I; the nucleus of a muscle cell 

 {or myoblast) ; I, gland cell ; o, oblique or diagonal umsclefe ; d, e, /, 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/cnstrasse 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-epidermal rhabdite cell, such as occurs in other 

 forms ; c', 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 Convoluta (from v. Gr.). s, the sagitta. 



9. The same, discharging its sagitta. 



10. A nematocyst from the Rhabdocoelid Microstoma 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. 



