138 COMPARATIVE ANATOMY CHAP. 



are flat and compressed. Most of the endoparasitic Distonm species, 

 on the contrary, are more or less cylindrical. The Ccstoda or tape- 

 worms are ribbon -like, and consist chiefly of a row of consecutive 

 segments progressively increasing in size posteriorly (proglottides). In 

 front, where the body thins away, the segmentation is indistinct. The 

 thin neck portion swells into a knot-like tapeworm-head (scolex), which 

 is provided with organs of adhesion. 



III. The Outer Body Epithelium. 



A distinct body epithelium occurs only in the Turlellaria. In the 

 Trematoda and Cesfoda it is replaced by a resistant, elastic, cuticular 

 membrane, which is generally perforated by very fine pores. It is 

 indeed not impossible that this integument itself is a much modified 

 epithelium. The body epithelium of the Turlellaria is ciliated. The 

 ciliary motion serves principally (1) for Respiration (by the main- 

 tenance of a constant circulation of the water over the naked surface 

 of the soft animal) and (2) for Locomotion (especially of the smaller 

 forms). 



Unicellular dermal glands open in the skin. In special glandular 

 cells of the skin the so-called rod or rhabdite cells, spindle-shaped 

 glandular secretions occur ; the so-called rods or rhabdites, although 

 found in other divisions, are specially characteristic of the TurbcUaria. 

 Whereas the rod-glandular cells in most Polyclada lie in the epithelium 

 itself, in most Tridada and PihaMoccelidce they sink deep into the 

 parenchyma, and only remain connected with the epithelium by means 

 of long thin processes (rod -passages) in which the rhabdites are 

 conveyed outwards and deposited in the epithelium. Typical stinging 

 capsules also occur in the epithelium of many Turlellaria. There are 

 besides numerous intermediate forms between true rhabdites and true 

 nematocysts. 



IV. The Gastro-eanal System. 



This has entirely disappeared in the Oestoda, These endoparasites 

 are nourished endosmotically by the juices which surround them. 



In the other Platodcs the gastro-canal system is well developed, and 

 shows in a few divisions modifications almost as important and 

 characteristic as those in the Ciridaria. We can everywhere dis- 

 tinguish in it two principal parts: (1) an oesophagus or pharyngeal 

 apparatus, which comes from a depression of the outer integument and 

 is consequently lined with ectodermal epithelium ; and (2) the endo- 

 dermal intestine or gastral apparatus. We will treat of these two 

 parts separately. 



I. The pharyngeal apparatus. This opens externally through 

 the mouth, and internally through an enteric aperture into the gastral 

 apparatus. 



