FLA T- WORMS. 685 



FLAT- WORMS. 



Class Platyhelminthes. 



Many of the worms of this division have been greatly modified by a parasitic 

 life, have lost all trace of the mouth and alimentary canal, and in the case of 

 the Tape-worms have acquired a rich segmentation of the body, which, as 

 will be explained, is subservient to the process of reproduction ; but the 

 forms that lead a free existence, and may be looked upon as the primitive 

 members of the class, approach the Nemertines in many points of structure, 

 though, on the whole, they are much less highly organised, there being no 

 distinct vascular system, no dorsal proboscis and sheath, and the alimentary 

 canal, when present, has no posterior outlet. 



The Planaeian Worms— SuB-CLAga Tukbellauia. — To this sub-class 

 belong the free-living flat-worms alluded to above. The flattish unseg- 

 niented body is either short and broad or long and slender, and, as in 

 the Nemertines, its integument is covered with cilia. The mouth, which 

 varies much in position, being sometimes placed at the front end of 

 the body aiid sometimes in the middle of its lower surface, leads into a 

 muscular pharynx, and this usually passes into a spacious digestive tract, 

 upon the structure of which the classification of the group is based. In the 

 first section, named Rhabdocosla, for example, the alimentary canal consists 

 of a single undivided pouch. In the Dekdroccela, on the other hand, the 

 stomach is branched, being divided into three (Tricladida), or a very large 

 number of branches (Polycladida), while in the section known as Aciela 

 there is no true intestine, the alimentary canal being merely represented by 

 the mouth and the muscular pharynx or gullet which immediately follows 

 it. Tn some cases, as represented in our figure of Bijxdium keweiise, one of 

 the land Planarians, the pharynx is capable of protrusion through the 

 aperture of the mouth, and of acting as a sucker. The nervous system is 

 well developed, and consists of a bilobed mass or ganglion placed in the 

 head, and from this a pair of widely-separated chords pass backwards to the 

 hinder end of the body, and others go to supply the head and eyes, of 

 which there are sometimes many, though more usually only one or two 

 pairs. Sometimes, too, ears of a simple kind are found. 



The habitat of Planarian worms is tolerably varied, some species being 

 found in the sea, others ill fresh water, and others, again, on the land. 

 Most of them are free living. A few, however, have taken to a parasitic 

 life, two of the Rhabdoccela, named Graffila and Ano^dodinm, occurring 

 respectively upon Mollusca of the snail-kind and upon Sea-cucumbers, 

 while Bdellura, a Triclad Dendroccel, lives on the king-crab. All the mem- 

 bers of the Polyclad Dendroccels are marine, and in this group we meet 

 with a metamorphosis in the development, the young Planarian, known 

 as Miiller's larva, being furnished with eight ciliated processes arranged 

 round the middle of its body. Thus equipped it leads a free life, circling 

 about in the water. In other members of the class the development takes 

 place without the intervention of active larval forms. Usually the method 

 of reproduction oflFers nothing worthy of special comment; but one of the 

 Rhabdoccels, known as Microstoma, presents an alternation of generation such 

 as has been described in the case of some Annelids (e.g., Nereis). The hinder 

 portion of the body becomes divided by a partition from the front portion; 



