686 VERMES CLASS PLATYHELMINTHES. 



each part then again divides into two, the process being repeated until there 

 may be as many as eight, sixteen, or thirty-two divisions. Each division 

 acquires a mouth, and when each is sexually mature, the chain breaks up 

 into a number of separate individuals, which lay eggs, these ultimately giving 

 rise to sexless individuals, which can only reproduce by fission. 



Many Planarians are elegantly coloured, presenting either a tolerably 

 uniform vivid tint, or considerable diversity of pattern. Two species, 

 namely, Vortex mridis and Convoluta tckuUeu, the former belonging to the 

 RHABDOCCELA and the latter to the ACOSLA, are noteworthy in this connection 

 as being coloured with chlorophyll, the material which imparts to plants 

 their characteristic green tint, and which enables them under the influence 

 of sunlight to break up carbonic acid gas into its constituent chemical 

 elements, oxygen and carbon. Both of these Planarians are of small size, 

 measuring less than half an inch in length. Many other kinds are, however, 

 much larger than this, one of the terrestrial species, Geoplaua australis, from 

 New Zealand, being upwards of a foot long. 



The acquatic forms swim by means of the undulatory movement of the 

 body, aided by the sweeping action of the multitude of cilia which clothe the 

 integument. The terrestrial forms progress in the same manner, preserving 

 their moisture by the copious secretion of slime. Avoiding the lighband living 

 under stones and logs of wood during the daytime, they venture forth at night 

 from their dark humid haunts, and crawl about with the sensitive head 

 raised and gently swayed from side to side to feel the way. Their food 

 consists of earth-worms and defenceless insects, the soft parts of which are 

 imbibed by means of the gullet, the prey being securely held the while by 

 the slimy coils of the Planarian. The figure of the Planarian (Bipalium 

 Jcewense) on page 684, represents one of these terrestrial leech-like species, 

 which is recognisable by the peculiar shape of its head. It was first 

 obtained in Kew Gardens, and has been introduced into many parts of the 

 world in connection with imported exotic plants. 



THE TAPE-WORMS SUB-CLASS CESTODA. Almost all the members of this 

 class live, when mature, as parasites in the alimentary canal of carnivorous 

 vertebrated animals ; but during the younger stages they occur in different 

 parts of the body of animals of many kinds, including ticks, fresh- water 

 shrimps, molluscs, and water-worms, as well as fish, rabbits, pigs, etc. 

 Unless, in fact, the worm is able to change its host it is quite unable to 

 reach the adult state. The truth of this is well illustrated in the case of one 

 of the commonest of mankind's Tape-worms, known as Tcenia solium, which 

 may be selected as a convenient type to illustrate the life-history and the 

 structure of the other members of the class. 



Upwards of three yards in length, this worm floats in the intestinal juices, 

 maintaining a firm hold of the inner wall of the gut by means of its minute 

 pin-sized head, which is furnished for the purpose with a circlet of five hooks 

 and four strong suckers. Immediately behind the head comes a narrow part, 

 called the neck, which is unjointed and only about half an inch long. This 

 is continuous with the rest of the body, which gradually expands into the 

 characteristic tape-like form, and becomes divided into a vast number of 

 segments. At first imperfectly defined, these segments become longer and 

 longer and more and more distinct as they pass from the neck to the posterior 

 end, the last ones alone being what is called "ripe" and ready to break off 

 from the parent stock. The worm is furnished with a well-developed muscular 



