OTHER ECHINODERMS 141 



(c) The living eggs can be easily fertilized in a dish of sea water 

 by addition of sperm and the development followed hour by hour. 

 Instead of such living stages stained material may be used, either 

 permanently mounted on slides or distributed in clearing fluid for 

 temporary mounting as directed by the instructor. In the one-cell 

 stage, or zygote, note the fertilization membrane, upon which the 

 heads of many spermatozoa that failed to enter the egg may be visible. 

 The two-cell, four-cell, eight-cell, later cleavage stages, and the blas- 

 tula stage, will be recognized. Find late blastula stages, showing the 

 origin of mesenchyme cells at one pole. Is the wall of the blastula 

 of uniform thickness? Can you recognize, even in the blastula, the 

 region that will invaginate during gastrulation to form the archenteron 

 of the gastrula? The opening of the archenteron is the blastopore. 

 After gastrulation the cells that have remained on the outside are 

 known as ectoderm, and those that line the archenteron are known 

 as endoderm. The mesenchyme cells and other cells which arise from 

 the inner end of the archenteron constitute the mesoderm. In this 

 manner the germ layers are formed. Make a series of outline figures 

 illustrating these developmental stages; size, 5 to 10 X the measured 

 size projected to table level. 



(d) As development proceeds, the blastopore of the gastrula be- 

 comes the anus of the larva. The mouth of the larva is formed by 

 an invagination, the stomodeum, which unites with the blind end of 

 the archenteron. A larva which is strikingly bilateral and which is 

 called a bipinnaria is formed within a day or two depending upon the 

 temperature (Fig. 64) . This larva becomes further specialized with 

 respect to its free-swimming existence near the surface of the ocean. 

 Other types of echinoderms have comparable bilateral larvae. From 

 these larvae, by a curious metamorphosis, the radially symmetrical 

 adult echinoderms are formed. The existence of such larvse constitutes 

 the main evidence for the belief that the present radially symmetrical 

 echinoderms have descended from bilaterally symmetrical ancestors. 

 Compare with the inferences drawn from the existence of fishlike 

 stages in the frog and chick. 



OTHER ECHINODERMS 



Exercise 1. — Other Starfishes. 



(a) Other species of starfishes should be examined with reference 

 to modifications of the body outline, skeleton, and number of rays. 

 This study should be supplemented by the examination of chart and 

 textbook figures. 



