BUTHUS. 159 



6. Under the operculum are the gills. How many groups 

 are there? Are they arranged in pairs? How are they attached 

 to the body? Are they movable? What reason is there for 

 moving them? Examine a bunch of gills, frequently called a 

 gill-book, and see how it is formed. 



7. At the base of the caudal spine notice the anus. 

 Make a drawing of the ventral surface. 



Internal Anatomy. This shows no very special adaptation 

 and can be pretty well understood by studying a longitudinal 

 section of a small preserved specimen. 



In such a section the following organs may be found: 



1. The dorsal extensor, the ventral flexor, arid the leg muscles. 



2. The elongated tubular heart just beneath the dorsal cover- 

 ing, in the posterior end of the cephalothorax and the anterior 

 end of the abdomen. 



3. The alimentary canal, consisting of the esophagus and the 

 anterior and posterior portions of the stomach, which extends 

 posteriorly without much change to the anus. The liver, which 

 surrounds the stomach and fills the greater portion of the 

 cephalothorax, sends its secretions to the stomach. 



4. The cerebral ganglia, near the bases of the chelicerse, and 

 the ventral chain of ganglia should also be .seen in satisfactory 

 sections. 



A drawing is desirable. 



Lankester: Limulus an Arachnid. Quart. Jour. Mic. Sci., 21, 1881. 

 Packard: The Anatomy, Histology, and Embryology of Limulus poly- 

 phemus. Mem. Bost. Soc. Nat. Hist., 1880. 



BUTHUS. (Scotpton.) 



Living specimens of these animals are not usually available 

 for laboratory study. They live for the most part concealed 

 during the day in crevices and holes and are active at night. 

 Their food is largely spiders and insects which are seized by the 

 claws and killed with the abdominal sting. 



1. Into what parts is the body divided? How many seg- 

 ments are recognizable? Which are the most freely movable? 



