154 PLATYHELMINTHES. 



The outer surface of the body is covered by a thick 

 J cuticle, underneath which is a layer of 



' ectoderm in which the cell-walls are said 

 to be absent. This ectoderm is thickened in the mid- 

 dorsal, mid-ventral, and the two lateral lines, corresponding 

 to the four external lines. Below the ectoderm is a single 

 layer of longitudinal muscle-cells, divided into four sections 

 j^ by the four ridges of ectoderm. Each muscle- 



cell has an outer muscular part with longi- 

 tudinal striation and an inner protoplasmic part with a 

 nucleus. As in Hydra, only a portion of the muscle cell is 

 differentiated into contractile tissue. 



The nervous and excretory systems are best seen in 

 sections. The former consists of a nerve-ring round the front 

 Nervous ^^ ^^^ pharynx which is thickened dorsally and 

 ventrally. Six small nerves run forwards and 

 six others run backwards. Of these the four lateral soon 

 become very thin, but the dorsal and ventral run the whole 

 length of the body, embedded in the ectodermal ridges. 

 They are connected by alternate lateral commissures. 



In the lateral ectodermal ridges there runs a pair of 



excretory ducts which apparently end blindly behind, but 



Excretory ^^^^ ^^ front to open by the median ventral 



pore a little behind the mouth. 



The male sexual organs consist of a single long coiled 



tube. The blind and tapering end forms the testis, the 



middle part the vas deferens, and the lower part swells out 



to form the seminal vesicle. This opens by a small duct into 



the intestine close to the anus. A small setal gland secretes 



the anal setae. The female organs consist of a pair of long 



coiled tubes. The inner part of each forms the ovary, the 



middle portion the oviduct which swells out to form the 



uterus. The two uteri join in a common vagina to open 



to the exterior by a median ventral opening towards the 



anterior end of the body. 



The fecundity is enormous, many thousands of fertilised 



and encapsuled eggs being discharged daily from the uteri. 



DeveloDme t These pass out of the body of the host, but 



their subsequent history is unknown. It is 



said that, as in the case of liver-rot, the maw-worm is 



acquired by feeding on damp pasture. 



