PROTOPLASM AND SIMPLE ANIMAL FUNCTIONS 467 



two-phased nerve-control over this heart is at least exceedingly prob- 

 able. 



Expt. 13. Respiration is carried on in Daphnia both directly by 

 diffusion through the thin membranous, chitinous shell and by means of 

 feathery gills attached to the four to six pairs of degenerate legs in the 

 ventral middle of the body, when seen on its side. The former means is 

 probably much the more important, the flatness of the animal, its small 

 size, and the broad exposure of blood immediately under the shell making 

 easy the direct exchange between the blood and the surrounding water. 

 The movements of the gill-feet are, however, probably a respiratory 

 reflex for waving the gills rapidly through the water, the shell being open 

 on the ventral side. Occasionally the abdominal end may be seen to 

 vigorously extend for the purpose of removing particles which have 

 entered the shell and are interfering with the feet's free activity. Crush 

 a Daphnia and study the freed gills with a higher-power objective. 



Expt. 14. Nutrition (including the preparation of food for the me- 

 tabolism of the tissues) is mechanically a simple matter in Daphnia. 

 Chemically, however, it may be very complex, for the food digested is 

 largely proteid. The fine hair-like antennule near the end of the beak 

 can be clearly made out, but whether it is an organ of taste, smell, or 

 touch is not known. The mouth-parts are not easily seen. Observe 

 the conspicuous quick peristalsis of the esophagus. Note the digestive 

 gland at the summit of the anterior bend of the greenish-yellow gut close 

 to the brain; this probably secretes the enzymes which dissolve the food. 

 It pulsates as a continuation of the peristaltic waves which pass up the 

 intestine. Sop a speck of cotton bearing croton oil (poison) on the 

 animal. This soon causes a marked increase in the intestinal movements. 

 Observe now the pulsations of the digestive gland. The peristalsis of 

 the gut is an antiperistalsis and now, exaggerated by the oil, the waves 

 may be seen to start at or near the anus and to pass in a typical way up 

 the gut as far as the enlargement opposite the heart, but here a less 

 definite surging sort of movement takes the place of the true peristalsis. 

 Study the peristalsis carefully, measuring the speed of the waves, their 

 frequency, etc. Within ten or fifteen minutes after administering the 

 croton oil the entire gut is usually free of its former contents. Note the 

 mode of defecation. 



Expt. 15. Nervous Function and the Senses. Note the brain, the 

 optic lobe, the nerve going to the antennules, the trunk extending from 

 the brain. According to Lang, there is a ladder-like ventral cord con- 

 sisting of seven pairs of ganglia, the foremost of which controls the 

 mandibles and maxillse, and the remainder the six pairs of legs. 



Demonstrate the presence of nerves inhibiting the heart by a light 

 puncture with a very fine bent needle over the caudal bend of the gut. 

 The heart stops instantly. Oftentimes in a few minutes it begins to beat 

 again (as does the mammalian heart after stimulation of the vagus). 



The compound eye of Daphnia consists of five simple eyes or omatidia. 

 The ocular muscles should be studied carefully and the movements which 



