STRUCTURE OF ASCIDIANS. 391 
Chial sac (A) to the stomach, while the intestine (B) is flexed, 
directed upwards, ending at the bottom of the atrium not 
far from the atrial opening. The reproductive glands are 
situated behind or below the bend of the intestine, the eggs 
being fertilized as they pass into the atrium, and the heart 
lies in the bottom of the body-cavity, being directly opposed 
to the nerve-ganglion (not represented in the figure), which 
lies between the two openings. 
In the perfectly transparent Perophora, which grows on 
the piles of wharves on the coast of Southern New England, 
one individual after another buds out (as also in Clavellina) 
from a common creeping stalk like a stolon. In this form 
the circulation of the blood-disks in the branchial vessels and 
the action of the heart can be studied by placing living ani- 
mals in glasses under the microscope. The heart is a straight 
tube, open at each end, and situated close to the hinder end 
of the branchial sac. After beating for a number of times, 
throwing the blood with its corpuscles in one direction, the 
beatings or contractions are regularly reversed and the blood 
forced in an opposite direction. 
Renal organs are apparently represented in Phallusia by 
a peculiar tissue, consisting of innumerable spherical sacs 
containing a yellow concretionary matter. In Molgula and 
Ascidia vitrea Van Beneden, an oval sac containing concre- 
tions of uric acid lies close to the ovary. 
In the forms already considered the plan of structure is 
complicated, owing to the difficulty of distinguishing an 
anterior or posterior, a dorsal or ventral aspect of the 
animal. In Salpa and Doliolum, however, the body is more 
or less barrel-shaped, the hoops of the barrel represented by 
the muscular bands which, at regular intervals, surround the 
body. The mouth is near the centre of the front end, the 
pharyngeal sac is very large, and the digestive tract makes 
less of a turn than in the ordinary Ascidians, while the 
atrial opening lies directly at the posterior opening. The 
heart is truly a dorsal vessel, and the nervous ganglion is 
situated on the opposite side of the body. This relation of 
the anatomical systems is most clearly shown in the genus 
Doliolum, and we have here a slight approach to the sym- 
