ON THE DEVELOPMENT OF LTNGULA ANATINA. 23 



combined action of the anterior growth and the expansion of the 

 arm- apparatus. It is not difficult to observe the process of bursting 

 under a microscope. A little opening being made at the tip of 

 the arm-apparatus the latter stretches out of the hole, which is 

 thus enlarged by degrees. Now the cilia on the arm-apparatus, 

 12 ft- in length on an average, move more rapidly than before. 

 The vitelline membrane is sometimes found for a long time at- 

 tached closely to the posterior half of the embryo. That the 

 vitelline membrane has persisted without rupture until now shows 

 on the one hand that the limit of elasticity of the membrane is 

 great, and on the other that the embryo has increased but little 

 in volume from the egg. This will best be seen on comparing 

 PI. I., Fig. 1., and PI. III., Fig. 44. By the longitudinal growth 

 of the arm-apparatus it becomes up-lifted on a short stalk, thus 

 simulating a mushroom (PI. III., Figs. 44, 45.). Concurrently 

 with this change the arm-apparatus becomes triangular with 

 rounded angles. The median angle is the Anlage of the tentacle 

 (tnt.), and the lateral two those of the first pair of cirri (cr r .). 

 Reaching this stage the embryo begins to contract its arm- 

 apparatus. That it is very sensitive is proven by the fact that 

 it instantly retracts its arm not only at a shock made on the 

 stage of the microscope, but even at a slight knock upon the 

 table. Enfeebled embryos, on the contrary, do not retract their 

 arm-apparatus at all. This may readily be observed even in far 

 advanced larvœ. When contracted, the longitudinal axis of the 

 arm-apparatus lies nearly parallel to that of the embryo, but 

 when fully extended the former makes an angle with the latter. 

 The semicircular form of the mantle becomes more pronounced, 

 coming to possess a straight posterior edge. The mantle at this 

 stage grows in all directions, that is, not only anteriorly and 



