ARTHROPODA — CRUSTACEA 283 



arate division of the compound eye gives a complete image, but the 

 resultant vision of the entire eye is not a mosaic, although each 

 facet gives a different perspective from its neighbor. The re- 

 sultant is a single image just as in man, where the two eyes, each 

 with a different perspective, produce a single image. 



Some delicate setae upon the shorter, or anterior, pair of 

 antennae are supposed to function as organs of smell. 



In the proximal segment of the shorter pair of antennae occurs 

 a sac-like inbending of the surface chitin. This sac, the otocyst 

 or statocyst, is in free communication with the water outside 

 through a small opening guarded by hairs. It is lined with 

 sensory feathered setae, similar in structure to the tactile setae, 

 and contains some minute sand grains. One function of this 

 organ has been shown to be the maintenance of equilibrium, 

 similar to that of the semicircular canals in man. Gravity 

 acting upon these grains brings them into contact with different 

 setae as the body becomes tilted at different angles and through 

 these setae corresponding sensations are produced in the neigh- 

 boring nerves which transfer them to the brain. Probably these 

 setae also take cognizance of the sound waves passing through 

 the water and thus act likewise as organs of hearing. When 

 the animal molts, the chitinous lining of this sac is also shed 

 so that new^ sand grains must be gathered after each molt. 



Reproduction is sexual. The eggs are of considerable size with 

 a large amount of yolk. They are fertilized immediately after 

 extrusion and are fastened to the swimming legs of the female 

 by a sticky secretion from glands on those appendages. 



The one-celled egg after fertilization rapidly develops into 

 the blastula-like stage and this into the gastrula. Thickenings 

 then develop in definite parts, forming the rudiments of the three 

 anterior appendages of the head; this is the nauplius stage. 

 After this the embryo rapidly passes into the form in which it is 

 hatched, that is, it becomes adult in form but not in size. 



1. What is the present habitat of Cambarus? 



2. Name the principal external divisions of the body. 



3. What are the divisions of a typical appendage ? 



4. Is the skeleton internal, i.e. covered upon the outside 

 by a renewing fleshy layer, or external ? 



