THE CRAYFISH *9 



toad by the transverse cervical suture. The abdomen 

 is composed of segments. How many ? The flattened 

 terminal segment is called the telson. Is the cephalo- 

 thorax 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 micro- 

 scope. 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 external window of an eye element or 

 ommatidium. An eye composed in this way is called a 

 compound eye. In front of the eyes note two pairs of 

 slender many-segmented appendages. The shorter pair, 

 the antenmdes, 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 con- 

 tains 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 in the fine hair- 

 like projections upon the joints is believed to be located 

 the sense of smell. Thus it will be 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 antenna. 



TECHNICAL NOTE. Stick one point of the scissors under the 

 posterior 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 



