30 THE ANIMALS AND MAN 



cervical suture. The abdomen is composed of segments. 

 How many? The flattened terminal segment is called the 

 lelson. Is the cephalothorax composed of segments ? Where 

 is the mouth of the crayfish ? Where is the anal opening ? 



At the anterior end of the cephalothorax note a sharp 

 projection, the rostrum. Where are the eyes? Remove one 

 of them and examine its outer surface with a microscope. A 

 bit of the outer wall should be torn off and mounted on a 

 glass slide. Note that it is made up of a great many little 

 facets placed side by side. Each of these facets is the exter- 

 nal window of an eye element or ommatidium. An eye com- 

 posed in this way is called a compound eye. In front of the 

 eye note two pairs of slender many-segmented appendages. 

 The shorter pair, the antennules, are two-branched. Remove 

 one of them and note at its base a small slit along the upper 

 surface. This slit opens into a small bag-like structure 

 which contains fine sand-grains. The bag is protected by a 

 series of fine bristles along the edge of the slit. This bag- 

 like structure is believed to be an auditory organ. The 

 longer pair of appendages are the antennce, and the sense 

 of smell is believed to be located in the fine hair-like pro- 

 jections upon the joints. Thus it is seen that the sense- 

 organs of the crayfish, like those of the toad, are located on 

 the head. Beneath the basal portion of each antenna there 

 is a flat plate-like projection, at the base of which on the 

 upper, edge will be noted a small opening, the exit of the 

 kidney, or green gland. 



Make a drawing of the surface of part of an eye; also of 

 an antennule; and of an antenpa. 



Technical Note. — Stick one point of the scissors under the pos- 

 terior end of the carapace on the right side, and cut forward, thus 

 exposing a large cavity, the gill-chamber. Remove all of the mouth- 

 parts, legs and abdominal appendages from the right side, being careful 

 to leave the fringe-like parts, the gills, attached to their respective 

 legs. Place all of the appendages in order on a piece of cardboard. 



