170 CHORDATA. 



or ''peribranchial" type. (In this type the outer vesicle arises 

 from the integument, and the inner vesicle from the parietal 

 wall of the atrial cavity.) The inner vesicle may be seen partly 

 constricted into three divisions — the pharynx and the two atrial 

 sacs. From which ''germ layer" then are these parts in the 

 bud derived ? 



5. Look for the tailed larvse or ''tadpoles" near the surface 

 and on the side turned toward the light, in a dish in which Bot- 

 ryllus has been kept for an hour or two. Is this positive photo- 

 trophism advantageous ? Examine a larva under a microscope. 



Drawings of the adult, the young colony, and the larva are de- 

 sirable. 



AMAROECIUM. (Sea-Pork.) 



Different species of this composite ascidian live at different 

 depths and show minor structural differences, especially in the 

 tests. Colonies may be found abundantly on piles and they are 

 frequently brought up with a dredge. 



1. Compare the grouping of the individuals in the colony 

 with Botryllus. Is there any regularity in the number of a 

 group connected with a common cloacal cavity? 



2. With a sharp knife, cut a section vertical to the surface of 

 the mass, and two or three millimeters thick, and study it with 

 a low power of the microscope. Other pieces should be squeezed 

 in a finger-bowl half full of sea-water, the expressed material 

 (adult animals, fragments, embryos, etc.) allowed to settle, 

 and then rinsed with clean sea-water. A few entire adults may 

 be picked out with a pipet. 



In the adult animal you may find: 

 (a) Oral and atrial openings. 



(h) Pharynx, with the peri-pharyngeal bands and endostyle, 

 esophagus, the orange-brown corrugated stomach, and intestine. 



(c) The cerebral ganglia. 



(d) The long post-abdomen, with its hollow epicardium con- 

 nected with the pharynx. (The post-abdomen is really a stolon. 

 Recall Perophora.) If complete, the red-pigmented tip will be 

 seen. 



