346 CHARACTERS OF INVERTEBRATE ANIMALS 



where it bites against its fellow. The first maxilla on either 

 side has a two-jointed stalk, upon the end of which are borne a 

 slender outer and a shorter inner branch. The former is made 

 up of a number of narrow joints, and is known as a palp, while 

 the inner branch consists of two parts, a cutting blade next the 

 middle line and a soft piece adjacent to the palp. The smaller 

 second maxillce are built on the same lines, but are fused together 

 to form what is commonly called the " under lip" or labium. It 

 is the united stalks which have so fused, and though these par- 

 ticular appendages are considered as jaws, the union has of course 

 taken away any power of biting against ^ach other. Projecting 

 from the front of the mouth is a broad horny plate, the "upper 

 lip " or labrum, and in the narrow space between this and the 

 labium the mandibles and cutting parts of the first maxillae work 

 against one another. The Cockroach is clearly an example of 

 Insects with biting mouth-parts, but in other cases we find 

 the corresponding appendages adapted to very different uses, 

 furnishing one of the best examples known of modifications of 

 a common type. 



A large kidney-shaped eye will be seen on each side of 

 the head, and a good lens will show that each of these possesses 

 a large number of six-sided facets. An eye like this is generally 

 known as a " compound eye ", each of the facets having been 

 formerly regarded as equivalent to an independent simple eye. 

 Some of the other external characters are mentioned elsewhere. 



Skin and Exoskeleton. The body is invested in a strong 

 horny covering secreted by the underlying epidermis. Move- 

 ment is rendered possible by the presence of softer areas between 

 the firmer tracts, so that the head is not immovably fixed, the 

 segments of the abdomen can move one upon the other, and 

 the joints of the limbs can be bent in various directions. In 

 fact the same problem has had to be solved as that involved 

 in the construction of a suit of armour, i.e. the combination of 

 efficient protecting power without undue sacrifice of flexibility. 

 It is of especial importance to notice that the narrow side of 

 the body is for the most part provided with a softer investment 

 than the broad upper and lower surfaces. 



Digestive Organs (fig. 202). The head of the Cockroach 

 is bent downwards at right angles to the long axis of the body, 

 so that the mouth-opening at its end faces downwards. It leads 



