APPENDIX. 413 



tacles down the side of the body. Near the upper end of the in- 

 testine will be found two large Polian vesicles, which form 

 part of the water-vascular system. The ovary is a bunch of 

 tubes attached to the end of the oviduct. 



It will be best to study the test or skeleton of the Sea-urchin 

 before examining the internal parts. The test may be freed from 

 the soft parts by soaking it for a few hours in a weak solution 

 of potash, then brushing away the softer portions with a bristle 

 brush (Figs. 96, 9V). The arrangement of the ambulacral and 

 interambulacral areas, the structure of the mouth-parts, the po- 

 sition of the ovarian and ocular plates, and the arrangement of 

 the skeletal plates should be studied. Note also the tubercles 

 on the plates and on the complete animal ; note the shape, po- 

 sition, and arrangement of the spines and ambulacral feet (Fig. 

 214). For the study of the internal organization one shell may 

 be opened longitudinally and another transversely (Fig. 28), 

 or specimens may be soaked for a day or two in two per cent, 

 nitric or chromic acid, which will remove the lime from the 

 test, leaving it soft and pliable (Fig. 39). 



YERMES. The Earthworm may be taken to represent this 

 group. Use the largest specimens obtainable. They may usu- 

 ally be found in the warm evenings of early summer, stretched 

 out of the burrows, on the lawn or in the garden. Study their 

 method of locomotion, the manner in which the burrow is made, 

 also how the food is grasped and swallowed. If not conven- 

 ient to do this out-of-doors, put several specimens in a flower- 

 pot or box of earth and study them in the laboratory. Read 

 the account of their habits in Darwin's " Vegetable Mould and 

 Earthworms." 



Earthworms may be killed by being put for a few minutes 

 into lukewarm water. Then transfer to flat dishes, which are 

 long enough to allow of extending the, specimens at full length. 

 Pour over them two to four times their bulk of fifty per cent, 

 alcohol and leave for six or eight hours, then place in seventy-five 

 per cent, alcohol for the same length of time. If desired, they 

 may be still further hardened by treatment with stronger alco- 

 hol. With regard to their external anatomy, note their shape, 

 the slight distinction between the anterior and posterior ends, 

 the segmented structure, the grouping of the segments into re- 

 gions anterior, girdle, and posterior the fairly constant num- 

 ber of segments in the first two regions, the cuticle covering the 



