THE PHYLUM PLATYHELMINTHES 213 



-Mouth-/ M m ^^ i^i^f 



fit 



Acoela. Rhabdocoela. Polycladida 



Figure 11.10. Other orders of the class Turbellaria. An example of the order 

 Tricladida is shown in Figure 11.1. 



order Tricladida, in which the intestine has three primary branches. 

 This is mainly a tresh-water group, but it also includes a few marine 

 and terrestrial flatworms. In the order Polycladida the intestine has 

 many primary branches. These worms are all marine. The Rhabdocoela 

 have an anterior mouth and a simple, straight intestine, while the 

 Acoela have no intestine at all. Rhabdocoels are common in all waters 

 and include a large number ol small species. The acoels are marine and 

 minute. Most of them are very sluggish. They have a ventral mouth 

 that opens directly into a mixture of mesenchyme and endoderm cells. 

 Bits of food are swallowed and phagocytized by the endoderm cells. 



79. Class Trematoda 



Trematodes are parasitic flatworms that attach to the host by means 

 of suckers (Fig. 11.11), and in which the entire adult epidermis has been 

 replaced by a cuticle (Fig. 11.12). The digestive, excretory, muscular and 

 reproductive systems are similar to those of the Turbellaria. The class 

 is divided into two primary groups, the Monogenea, having a life cycle 

 involving only a single host, and the Digenea, having a life cycle in- 

 volving two or more kinds of host. 



The Monogenea are mostly ectoparasitic, living on the external 

 surface of the host. They have one or more adhesive organs next to 

 the mouth and one or more posterior suckers, with which they creep 

 about like inchworms (Fig. 11.11). This group includes the gill flukes, 

 common on the gills of marine and fresh-water fishes. Following copu- 

 lation, which is much like that in Dugesia, the hermaphroditic adults 

 lay eggs, one to a capsule, at the rate of several to 150 per day. These 

 have a thread on one end by which they become entangled on the sur- 

 face of the host or in the vegetation. They hatch in a week to a month 

 into small larvae that resemble the parent except that they are clothed 

 in a ciliated epidermis and have less elaborate attachment organs. By 

 means of the cilia the larvae swim to the appropriate host. Maturation 



