II.] THE CRAYFISH AND LOBSTER. 1 93 



«. The short two-jointed basal portion {protopo- 

 dite), consisting of a shorter proximal and a 

 longer distal piece. 



/J. The flattened lamellae attached to the distal 

 joint of the protopodite, an inner (endopodite) 

 and outer (exopodite). 



2. The structure of the cephalothorax. 



a. Note again the carapace, with its. frontal spine and 

 cervical suture. 



b. Turn the animal over and note the narrow sterna 

 of the thorax; they are laterally compressed, as the 

 result of the great development of the basal joints 

 of the appendages. 



The sternum of the last thoracic somite is not 

 completely ankylosed with the one in front in the 

 Crayfish. In the Lobster it is. 



c. Raise with a pair of forceps the free edge of the 

 lateral expansion of the carapace which overlies the 

 bases of the thoracic appendages: note that it is 

 formed by the large united pleura of the thoracic 

 segments, and overlaps a chamber in which the 

 gills lie. It is termed the branchiostegite ac- 

 cordingly; its ventral edge is smooth in the Cray- 

 fish, it is notched metamerically in the Lobster. 



d. Make a transverse section across the thorax, im- 

 mediately behind one of the larger appendages; 

 boil, for five minutes in weak solution of Caustic 

 Potash and pick away the soft parts, until quite 

 clean. Examine under water, and note, as com- 

 pared with the corresponding section across the 

 abdominal segment — 



m. 13 



