TYPE PLATTHELM1NTHB8. 131 



which lie scattered about in the ectoderm between its compo- 

 nent cells or may project more or less beyond its surface. 

 These rhabdites, as they are termed, are produced as a secre- 

 tion by cells lying usually in the mesoderm and connected 

 with the exterior by a slender neck passing through the base- 

 ment-membrane, the rhabdites thus making their way to the 

 exterior. The rhabdite-cells are ectodermal, their position in 

 the mesoderm being quite secondary, and in fact in one group 

 they are confined to the ectodermal layer. The function and 

 nature of the rhabdites have been variously interpreted, some 

 authors considering them equivalent to the Cnidarian nemato- 

 cysts, but it seems more probable that they are the condensed 

 secretion of cells which originally produced a mucous sub- 

 stance and by slowly dissolving in water produce a viscid 

 slime of sufficient tenacity to retain organisms coming in con- 

 tact with it. 



In addition to these structures many forms possess adhe- 

 sive cells, columnar cells which produce a strongly adhesive 

 secretion which is poured out in drops upon the free ex- 

 tremity of the cell, recalling in this respect the adhesive cells 

 of the Otenophores. These cells seem to be of use mainlj' in 

 enabling the worms to adhere to the surface on which they 

 are creeping, and are especially developed towards the hinder 

 end of the body. Another organ of adhesion in the form of a 

 muscular sucker, situated usually about the middle of the 

 ventral surface, is present in certain marine Turbellaria, but 

 the majority of the members of the group lack such struc- 

 tures. 



The nervous system consists of a brain from which a num- 

 ber of nerve-cords arise, varying somewhat in their arrange- 

 ment in the different orders. Sense-organs of one kind or 

 another are usually present in addition to the widely-distrib- 

 uted sensory cells of the ectoderm. A large number of forms 

 possess eyes, which in some Polyclads may be exceedingly 

 numerous, and usually consist of a patch of pigment lying in 

 the mesoderm and upon which a refractive lens-like structure 

 lies. In a few cases, as m' Microstoma, the eye is simply a patch 

 of pigment in the ectoderm near the anterior end of the body. 

 An otocyst, consisting of a spherical vesicle filled with fluid 



