STRUCTURE OF ASCIDIANS. 201 



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 tho 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 Peropliora, 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, tho 

 beatings or contractions are regularly reversed and the blood 

 forced in an opposite direction. 



Eenal 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 tho 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 DoUolum, 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 month 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 

 DoUolum, and we have here a slight approach to the sym- 



