INSECTS. 459 



marine polyzoa. This arrangement of the tentacles seems to be unique among the 

 fresh-water species, for although the tentacles of Fredericella appear at first to assume 

 the circular form, a more careful examination will show that this is not really the case. 

 The mechanism by which the floating particles contained in the water are inevitably 

 driven towards the mouth is seen in fig. O, which represents the tip of a single ten- 

 tacle greatly magnified, and is given in order to show the arrangement of the cilia, all 

 those of one side pointing upwards and those of the opposite side downwards. Fig. 

 N. represents the tentacular plume viewed from the front, and is given in order to show 

 that the arrangement of these organs is really circular. Only the bases of the tentacles 

 aie delineated. 



INSECTS. 



THE INSECTS, to which we must devote a few pages, afford the first examples of the 

 Articulata, /. e. the jointed animals without vertebrae. Their bodies are composed of a 

 series of rings, and they are separated into at least two and mostly three portions ; the 

 head being distinct from the body. They pass through a series of changes before 

 attaining the perfect form ; and when they have reached adult age they always possess 

 six jointed legs, neither more nor less, and two antennae, popularly called horns or 

 feelers. 



In most instances their preliminary forms, technically called the larva and pupa, are 

 extremely unlike the perfect Insect ; but there are some in which, at all events externally, 

 they retain the same shape throughout their entire life. The whole of the growth takes 

 place in the preliminary stages, so that the perfect Insect never grows, and the popular 

 idea that a little Insect is necessarily a young one is quite incorrect. It is true that 

 smaller and larger specimens occur in every species, but this difference in size is due 

 to some external influences that have acted on the individual ; and we find large and 

 small examples of an Insect, say a wasp, or a beetle, just as we find giants and dwarfs 

 among mankind. 



Insects breathe in a very curious manner. They have no lungs nor gills, but their 

 whole body is permeated with a network of tubes through which the air is conveyed, 

 and by means of which the blood is brought in contact with the vivifying influence of 

 the atmosphere. These breathing tubes, technically called tracheae, ramify to every 

 portion of the creature and even penetrate to the extremities of the limbs, the antennae, 

 and even the wings, when those organs exist. Their external orifices are called spiracles, 

 and are set along the sides. 



They have very little internal skeleton, the hard materials which protect the soft 

 vital organs being placed on the exterior, and forming a beautiful coat of mail, so 

 constructed as to defend the tender portions within, and yet to permit perfectly free 

 motion on the part of the owner. Certain projections of this substance are often found 

 in the interior, especially in the thorax, a central portion of the creature, and are used 

 for the attachment of muscles where considerable power is needed. 



This external skeleton is quite unique in its chemical composition, being made almost 



