254 NATURAL HISTORY. 



The genus Mermis has species which are parasitic within lusecta, arid at a certain time they 

 make their way out, by perforating their hosts, and hide themselves in the soil. They there reproduce, 

 and the embryos are born viviparously and pass some time in the ground. They wander in search of 

 an insect host, the caterpillar of a Tinea, or Moth, for instance, which they penetrate by means of a 

 sharp stylet, that is hidden within the head when not used. Mermis niyrescens emigrates en masse 

 out of insects, during hot weather, and being found on the ground in great numbers, gives rise to the 

 popular belief that " it rains worms." After this emigration, the embryos live in the pharynx of a 

 Planaria, The Humble Bee (Bombus terrestrits), and others of the genus, are the unwilling hosts of a 

 curious worm, one-fifteenth of an inch thick and an inch long, white in colour, blunt at either extremity, 

 .and covered with knobs, about 800 in number. This worm is the female of a species of Sphserularia, 

 and the male is 28,000 times smaller than the female, and is permanently attached to her. Another 

 family of these Nematoidea contains the genus Gordius, the embryos of which have a mouth, 

 and are found within the bodies of carnivorous water insects. They penetrate outwai-ds and get 

 into the water and become sexually mature. The embryos coming from their eggs penetrate the 

 larvae of water insects, such as those of Tipulidse and Ephemeridse, where they become encysted. 

 Then the carnivorous water larvse and beetles swallow the others, and of course take in the parasites 

 which rupture their cysts and live free in the visceral cavities of their bearers. 



The Anguillulidae are mostly non-parasitic Nematoid Worms, and some of them are known as 

 paste and vinegar eels. Others live in the mucous secretions of animals, and some are dwellers in 

 mushrooms. One gives rise to a diseased condition of the wheat ear. The young are hatched from 

 -eggs laid by the parent in the ear, and they become encysted. When the wheat dies down, the 

 larvse are set free and wander on the moist earth until they meet with some young wheat plants, up 

 which they creep and lodge themselves in the developing ears. Here they become sexually mature, 

 :and nourish themselves at the expense of the inflorescence. 



ORDER CH^ETOGNATHA. 



Allied to the last-mentioned family of the Nematoidea are the species of the genus Sagitta, which 

 .are associated in this order. They are long transparent worms with a special mouth armature 

 and pectinate fin-like feet, placed horizontally at the sides of the body, and their rays united by a web. 

 'The head is distinct, and has two sets of hooks which simulate jaws on each side of the mouth. They 

 swim freely in the sea, and live on small Crustacea. 



CLASS PLATHELMINTHA. THE FLAT WORMS. 



These are the most lowly organised Vermes. Many are parasitic within animals, and some live 

 in mud or in water, hiding under stones. 



They are divided into three orders, of which the first is that of the CESTOIDEA,* or Tape or 

 Ribbon Worms. 



The Tape Worms, which are parasitic within many vertebrate animals, including man, live in the 

 intestinal canals of their hosts, and are readily recognised by their long, flat, many-jointed bodies, 

 narrow and small heads usually armed with hooks and suckers suited for clinging on, and 

 gradually narrowing tail end. Some genera have species of enormous length, which consist of hundreds 

 -of joints or metameres behind the head, and others have the head and a hinder part not jointed and of 

 no great length. None have any digestive organs, the nutritious juices of the host passing into the 

 worms through their delicate integuments ; and no special organs of sense exist. In the Tape 

 Worms, both long and short, the head or scolex divides during growth behind more or less into a 

 joint or metamere, which is called a proglottis, and in this last the reproductive organs ai*e 

 developed, there being none in the head itself. As growth proceeds, the successive joints are given off 

 from the back part of the head, so that a long chain of them is produced, the oldest metamei-e being 

 that at the tip of the tail. All these metameres can produce ova. After a while, the time depending 

 upon the maturity of the egg-producing appai^atus, some of the metameres break off and are set free 

 from the rest of the worm, which still grows on as before. The growth of the metameres from the 

 "back part of the head is thus a kind of budding, and as each metamere, when detached from the 



* Greek, kestos, a girdle. 



