AMARCECIUM. 171 



(e) The slowly pulsating U-shaped heart, situated very near 

 the tip of the post-abdomen. 



3. In the atrium, which serves as a brood-pouch, embryos 

 in all stages may be found. How do the eggs compare in size 

 with those of Molgula? 



4. Look for buds formed by segmentation of the post-abdo- 

 men (stolon). The " inner vesicle" of these buds, which gives 

 rise to the alimentary canal and atrial sacs, comes from the 

 endodermic epicardium, as in Perophora. Compare this with 

 Botryllus. 



5. If the material squeezed in the finger-bowl was quite fresh, 

 living embryos in all stages of development can be found, and 

 the tailed larvae will probably be found hatching during the first 

 hour or two. These swim rapidly, and usually swim away from 

 the light. Does this correspond with Botryllus? Is this nega- 

 tive phototrophism adaptive? 



The tailed larvae may be picked up with a pipet while swim- 

 ming (the dead ones on the bottom of the dish should not be 

 used), dropped into fixing fluid, and finally stained and mounted 

 in balsam. Some larvae will be found attached to the dish by 

 their adhesive organs. Notice where these organs are sit- 

 uated. 



In larvae that have been previously stained and mounted 

 observe : 



(a) The shape of the animal and its division into body and 

 tail. 



(6) The thick test, and the oral and atrial openings. 



(c) The adhesive organs. How many are there? 



(d) The notochord. How far does it extend? 



(e) The tail muscles. 



(/) The pharynx, with as yet few gill clefts, the endostyle, 

 esophagus, stomach, intestine, and yolk-mass. 



(0) The cerebral vesicle with the eye-spot and otolith. 



If young individuals that have been attached but a short 

 time, but have lost their tails, are stained and mounted, they 

 will be found very instructive when compared with the larva. 



