84 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



or cuboidal entoderm cells, the line of mesogloea, and the profile view of the 

 ectoderm. The center of the tentacle is occupied by a slender canal, a branch 

 of the gastrovascular system, but this is distinctly identifiable only when food 

 particles in it can be seen moving back and forth. Draw a small portion to 

 show the cell arrangement in optical section. Then examine the more distal 

 (i.e., farther away from the body) portions of the tentacle to see the arrangement 

 of the stinging cells. Each group of stinging cells, or battery, is contained 

 within a single ectoderm cell, and causes a conical projection which appears 

 clearly when the ectoderm is viewed in profile. Note the definite arrangement 

 of large and small nematocysts in each battery and the projecting spines, the 

 cnidocils, best seen in profile. Draw a small portion of the tentacle to show 

 several batteries. 



4. The nematocysts. Remove the supports from under the cover glass 

 and drum on the cover glass with the point of a pencil until the animal is thor- 

 oughly crushed. This discharges the nematocysts. With the high power 

 examine undischarged and discharged nematocysts. There are usually three 

 kinds of them, a large barbed and two small non-barbed varieties. 



a) Large barbed nematocysts: In the undischarged state these are oval sacs, 

 with one end flattened. From the flat end a hollow pouch projects into the 

 interior, and from the inner end of this a coiled thread arises filling the rounded 

 part of the sac. When discharged, pouch and thread are projected to the exterior, 

 turning inside out. The pouch bears three large spikes and some quite small 

 ones (see Hegner, Fig. 56, p. 121). Some kinds of Hydras have two sizes of this 

 type of nematocyst. 



b) Small oval nematocysts: These are much smaller than the preceding, oval 

 or cylindrical, generally pointed at one end. When undischarged a spirally 

 coiled thread fills the whole interior, projecting inward from the pointed end. 

 When discharged the extremely long fine thread readily identifies this type. 



c) Small spherical nematocysts: These are a little smaller than the preceding 

 and more spherical. Each contains a thick thread which makes a single loop, 

 inside the nematocyst and fojrns a tight little coil of three or four turns when 

 discharged (see Hegner, Fig. 57, p. 122). 



Nematocysts are not really cells but cell products secreted by the interstitial 

 ceUs, which may frequently be seen clinging to them. Interstitial cells engaged 

 in forming nematocysts are called cnidoblasts, and from them project slender 

 spines, the cnidocils, whose stimulation is supposed to cause the explosion of the 

 nematocysts. 



Draw as many kinds of discharged and undischarged nematocysts as you 

 can find without spending too much time upon them. 



5. Cellular structure from slides. 



a) Longitudinal section: Examine slide " Hydra long." Find a nearly 

 median longitudinal section. Identify under low power the ectoderm, with 



