OF TUBULAR AND CELLULAR POLYPI, AND OF ASCIDI^E. 381 



nels were connected also by the smaller vertical passages between the spiracles ; the 

 set of the current in the latter being upwards for the two lower rows, and down- 

 wards for the two upper ones. 



Another large portion of the blood, on leaving the heart, immediately divided into 

 many ramifications, that spread like a network over the stomach and intestines, and 

 the soft substance of the mantle. Of these a part ran into the horizontal passages 

 above the branchial sac, a part into the descending back stream ; a large proportion, 

 after leaving the intestines, took a short course, and collecting into one channel, flowed 

 into that stream near the bottom; and all, united, then entered the peduncle, and con- 

 stituted the returning current that went to circulate in other animals of the group. 



After this circulation had gone on for a while, the pulsations became fainter for a 

 few beats, and the flow slower ; and suddenly, with but slight pause, the whole cur- 

 rent in all its windings was reversed. The heart gave the opposite impulse ; the 

 channel in the peduncle that before poured in the blood, now carried it back, and 

 the other the contrary ; and every artery became a vein. These changes continued 

 to succeed each other alternately ; the average time of the currents being the same 

 in both directions, but the period of each varying within a single observation as 

 much as from thirty seconds to two minutes. The phenomenon, like the currents in 

 the Sertularice, was invariably met with in every animal of the species that came 

 under my notice. 



Sometimes when the creeping tube or the peduncle has been injured, the circu- 

 lation of an individual is in consequence insulated, but without appearing to impair 

 any of its functions. I severed one at the part where it joined the peduncle ; when 

 for a few seconds the pulsation ceased ; it then began irregularly and with consider- 

 able pauses, and increased in steadiness as it went on. At first the impulse given 

 by the heart was towards the front ; and the downward back stream, instead of 

 flowing out at the wound, was poured into the hinder end of the ventricle, at n, fig. 2; 

 but when the current was reversed, part of the blood was driven for a time through 

 the stump of the peduncle into the water : however, it soon staunched, and all the 

 vital actions went on as before the separation, except that at the beginning of every 

 pulsation there was a slight recoil. 



. In one case, where the circulation did not extend to another animal, one channel, 

 and only one, was open in the peduncle, and in this a small current ran to or fro ac- 

 cording to the direction of the impulse given by the heart. Some animals, which had 

 probably been injured but were still connected with other vigorous ones, seemed to 

 be in course of absorption. One was observed in which the soft parts were so shrunk 

 as to occupy a small part only of the tunic ; the currents of its peduncle extended 

 into this mass, but no heart or motion of branchiae was visible. Upon looking at the 

 same the next day the tunic was empty, the soft matter and circulation reaching 

 only to the end of the peduncle. I also once noticed a flux and reflux of the blood 

 in a creeping stem, where the current did not communicate with any animal. 



MDCCCXXXIV. 3 D 



