INSECTS. 459 



marine polyzoa. This arrangement of the tentacles seerns 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. 0, which represents the tip of a single tentacle 

 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 are 

 delineated. 



INSECTS. 



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

 Articulata, i.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 antennas, 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 

 entirely of a substance called chitine, to which are added several other materials, such as 



