164 Phylum N em ertea 



Bay and on the coast of Maine. Several species of the same genus on 

 the Pacific coast from San Diego to Alaska furnish equally beautiful ova. 



The female Cerebratulus may be kept in a vessel of clean seawater for 

 3 weeks or more, if the water is changed daily and the temperature is held 

 at about io° C. But the eggs are less suitable for experimental work 

 after the first week. They are spawned spontaneously under such condi- 

 tions. 



To obtain large numbers of ova or young larvae it is necessary to free 

 the eggs from the body. This is best done by taking a small fragment of 

 the body, placing it in a dish of cool seawater and making a longitudinal 

 slit with sharp knife or scissors on the dorsal surface on each side of the 

 median line. The muscular contractions of the fragment will soon force 

 the ripe ova into the water. After a few minutes as many eggs as are 

 wanted are drawn into a pipette and expelled into a dish of clean seawater. 

 They are thereby washed free of most of the body fluids. 



The eggs on reaching the water still have the germinal vesicle intact 

 and are not yet ready for normal fertilization. Immediate fertilization 

 usually results in polyspermy. The stimulus of the water soon results in 

 the formation of the first polar spindle which proceeds to the metaphase 

 and then rests. This stage is reached in 10 to 30 minutes after the egg 

 reaches the water, the time depending both on the temperature and on 

 the ripeness of the eggs. The eggs are then ready for fertilization. 



To obtain the sperm a small fragment of a male (which may be dis- 

 tinguished from the female by its brighter color) is placed in a dish of 

 clean seawater and a puncture made through the dorsal body wall. 

 The sperm ooze out in a dense mass. A surprisingly minute quantity of 

 this, when expelled from a pipette into a dish containing the ova, will 

 suffice for complete fertilization. If larvae are desired the fertilized eggs 

 must be provided with a generous supply of clean, cool seawater. 



REARING LARVAE 



Prostoma and other hoplonemerteans develop directly into young 

 worms without the intervention of a free-swimming larval stage such as 

 is characteristic of Cerebratulus. In Lineus an intermediate condition 

 known as the Desor larva occurs. These larvae require no special feed- 

 ing, but the pilidium larva of Cerebratulus or of Micrura may be reared 

 to the adult form only by the most careful attention. 



The difficulty lies in providing suitable nourishment during the 20 or 

 more days which the swimming larva requires before metamorphosis is 

 completed. Small diatoms and other minute algae may be supplied 

 daily, with frequent changes to clean seawater. A more reliable method is 

 to feed regularly from a pure culture of the smallest obtainable diatoms. 

 The temperature of the water should not exceed 20 C. 



