VERMES. 53 



Myzostoma, an anomalous form, constituting the suborder Ecto- 

 parasita of Von Hayek, is found on Comatulce. 



Amphistomum. 



Polystomida. Diplostomum. 



Onchocotyle. Gastrodiseus. 



Myzostomldce. 

 Myzostoma. 



Order III. CESTODA. 

 T.ENIADA. AGASTRE^E. 



Internal parasites, mostly of an elongated and flattened form ; 

 the anterior end of the body or head provided with hooks or 

 spines, or both, or suckers. No digestive nor vascular systems. 

 Hermaphrodite. 



In the more typical forms these parasites are composed of a 

 head, which is the true animal, the joints being the hermaphro- 

 dite reproductive organs developed by a process of gemmation 

 from the head. The joints are called proglottides, and are orga- 

 nically connected by the water-vascular system. There is only 

 one proglottis in Caryophyllaeidae ; but in Tcenia solium there are 

 sometimes as many as 800. 



Although the proglottides are only produced in the alimentary 

 canal of man or some other warm-blooded animal, it is necessary 

 for the evolution of an embryo that the ovum should be swallowed 

 by some other animal than the one inhabited by the mature worm. 

 When the fecundated proglottides, therefore, are expelled, the de- 

 composing ova are liberated ; and should an ovum get into the 

 alimentary canal of a vertebrate, the embryo (now called a " pro- 

 scolex"), set free from its covering, proceeds to bore with its 

 spines through the tissues of its host until it finds a resting-place ; 



