270 PRACTICAL LESSONS IN SCIENCE. 



the second pair of jaws, and by its motion a circulation of water 

 is kept up through the gill chamber. 



When the foot jaws are removed two pairs of soft leaf-like 

 appendages, called jaws or maxillse,come into view, and anterior 

 to these are the mandibles, short, hard, toothed organs which 

 seem more like jaws than the softer maxillae. Each mandible 

 bears an appendage, which curves around its anterior border in a 

 groove, and is known as the mandibular palpus. Fitting closely 

 against the posterior margin of each mandible is a soft plate-like 

 organ called the metastoma. Remove each of these organs, not- 

 ing carefully its mode of attachment and its relations, experi- 

 menting with each so as to gain a clear idea of how the mouth 

 parts are arranged, and what part each plays in the process of 

 mastication. 



Just in front of the mandibles are the long feelers or antennae, 

 the antennulae, and lastly the eye stalks. Examine the an- 

 tennae; notice their mode of attachment, freedom of movement, 

 etc. In the basal segment of the antenna? there is an opening 

 leading to the green gland in the extreme front part of the body. 

 It serves the purpose of a kidney. In the basal joints of the 

 antennula? are the auditory organs; notice the auditory hairs 

 and the auditory sac. Examine the eye stalks and find the black 

 tip or cornea, and try if you can tell whether it is simple or com- 

 pound. 



In the larger crayfish the digestive, circulatory, and nervous 

 systems may be studied, but the dissections are best made under 

 water. Wedge a thin board into the bottom of a shallow dish, 

 or cover the bottom with wax, then placing the ventral surface 

 downward, pin the body firmly through telson and claws. Then 

 with a pair of scissors cut from the posterior margin of the cara- 

 pace forward a little to one side of the median line to the cervical 

 groove, then break away the whole side of the carapace. Push 

 away the gills and cut them off at the point of attachment, Cut 

 away the other side of the carapace. With the forceps clear 

 away the lining membrane which discloses a small chamber con- 

 taining a little whitish polygonal sac which is the heart. Notice 



