xii BACKBONELESS ANIMALS 237 



worm. The bladder part is of no importance, but the 

 head fixes itself and buds off a chain. For many others 

 the storv is similar ; the bladder-worm of the ox becomes 



, 



another tapeworm (Tcenia saginata] in man ; the bladder- 

 worm of the pike or burbot becomes another (Bothrio- 

 cephalus latus) ; the bladder-worm of the rabbit becomes 

 one of the tapeworms of the dog, that of the mouse passes 

 to the cat, and so on. A bladder-worm which forms 

 many heads destroys the brain of sheep, causing " sturdie ' 

 or staggers, and has its tapeworm stage (Tcenia ccenurus) 

 in dog or wolf. Another huge bladder- worm, which has 

 also many heads, and sometimes occurs in man as well as 

 in cattle, sheep, and pigs, etc., has also its tapeworm 

 stage (Tcenia echinococcus) in the dog. But enough of 

 these vicious cycles. 



2nd Set of Worms. Ribbon Worms or Nemerteans.- 

 Class : Nemertea. In pleasing contrast to the flukes and 

 tapeworms, the Nemerteans are free-living " worms." 

 Thev are mostly marine, often brightly coloured, almost 



/ / cu / 



always elongated, always covered with cilia. There is a 

 distinct food-canal with a posterior opening, a blood- 

 vascular system for the first time, a well-developed 

 nervous system, a remarkable protrusible ' proboscis ' 

 lying in a sheath along the back, a pair of enigmatical 

 ciliated pits on the head. The sexes are almost always 

 separate. Almost all Nemerteans are carnivorous, but 

 two or three haunt other animals in a manner which leads 

 one to suspect some parasitism ; thus Malacobdella lives 

 within the shells of bivalve molluscs. We find many 



/ 



of them under loose stones by the sea-shore ; one beautiful 

 form, Linens marinus, sometimes measures over twelve 

 feet in length. Some, such as Cerebratulus, break very 

 readily into parts, even on slight provocation, and these 

 parts are sometimes able to regrow the whole. We look 

 back to them with peculiar interest for several reasons, 

 one of which is that some of them show, for the first time 

 in the animal kingdom, haemoglobin, the blood-pigment 

 which makes the blood of backboned animals red, and is 

 of fundamental importance because of the ease with which 

 it enters into a loose chemical union with oxygen. Nemer- 



