TUNICATA. 



1217 



of the pulse were pretty regular in the same 

 individual ; but in different ones they varied 

 from two seconds to one and a half second. 

 Part of the blood thus impelled formed a main 

 upward stream along the front of the branchial 

 organ, branching off at each of the horizontal 

 passages between the rows of spiracles, and 

 traversing the space above them on a line with 

 the junction of the branchial sac and the mantle 

 on each side. All these streams again united, 

 and formed a downward current behind. The 

 horizontal vessels were connected also by the 

 smaller vertical channels between the spiracles ; 

 the set of the current in the latter being up- 

 wards for the two lower rows, and downwards 

 for the two upper rows. 



Another large portion of the blood, on 

 leaving the heart, immediately divided into 

 many ramifications, that spread like a net-work 

 over the stomach and intestines, and over the 

 mantle. Of these, a part ran into the hori- 

 zontal passages above the branchial sac, a part 

 into the descending dorsal vessel. A large 

 proportion, after leaving the intestines, took a 

 short course, and collecting into one channel, 

 flowed into the dorsal vessel near the bottom, 

 and all, united, then entered the peduncle, 

 and constituted 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 current was reversed, 

 the heart gave the opposite impulse. The 

 vessel in the peduncle that before poured in 

 the blood, now carried it back, and the other 

 channel 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. Mr. Lister points out the ana- 

 logy between this phenomenon and the very 

 similar circulation that obtains in the stem of 

 the Sertularia, described by him in the same 

 volume of the Philosophical Transactions. 

 This acute observer goes on to say, that some- 

 times, when the creeping tube, or the peduncle, 

 has been injured, the circulation of an indi- 

 vidual is in consequence insulated, but without 

 appearing to impair any of its functions. In 

 one animal which was severed from the pe- 

 duncle, the pulsation ceased for a few seconds. 

 It then began irregularly, and with consider- 

 able pauses, increasing 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 ; the cut end of the vessel leading 

 from the heart being nearly opposed to the 

 bleeding dorsal vessel cut through at the same 

 place, and the two vessels, in their undisturbed 

 state, lying across each other at this point. 

 But when the current was reversed, part of 

 "he blood was driven for a time through the 

 lump of the peduncle into the water; how- 

 ler, it soon staunched, and all the vital 



VOL. IV. 



actions went on as before the separation, ex- 

 cept 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 only 

 was observed to be open in the peduncle, and 

 in this a small current ran to and fro accord- 

 ing 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 the cir- 

 culation reaching only to the end of the pe- 

 duncle. Mr. Lister also once noticed a flux 

 and reflux of the blood in a creeping stem, 

 where the current did not communicate with 

 any animal. In the buds sprouting from the 

 stem, and destined to become new animals, 

 the two streams of the stem run through the 

 germ before its organs are developed. 



The generation of the Clavellmidte takes 

 place in two ways by ova and by buds. The 

 genesis of the larvae of Clavellina from the ova 

 is very similar to that of the larval Ascidue. 

 The young of Perophora have not yet been 

 observed. The method of the gemmiparous 

 reproduction, according to the observations 

 of Milne-Edwards, is as follows : Amongst 

 the radiciform processes springing from the 

 base of the test of Clavellina, and tending to 

 preserve the animal in its position, are other 

 filamentous prolongations which are hollow, 

 and enclose a membranous tube, continuous 

 with the internal tunic of the animal, and 

 through which the circulation seen in the in- 

 terior of the abdomen is also continued. This 

 stolon-like body is closed at the free extrem- 

 ity ; it is at first simple, but as it lengthens, 

 it becomes ramified ; and when its growth is 

 further advanced, there are developed on the 

 extremities of its branches, and even at dif- 

 ferent points along its length, tubercles, en- 

 closing in their interior a minute organic 

 mass connected with the interior tube. These 

 tubercles rise vertically, and become elongate 

 and claviform. The blood that circulates in 

 the stem at first penetrates into the soft, pear- 

 shaped, pedunculated mass occupying each of 

 the tubercles ; but after a while these little 

 germ-masses lose their peduncle and their at- 

 tachment to the internal tunic of the principal 

 canal, and participate no more in the circula- 

 tion of the mother-animal. By further de- 

 velopment they soon put on the ascidian 

 character. The branchial sac is perfectly dis- 

 tinct, although not yet communicating with 

 the exterior ; and the digestive tube bent 

 loop-wise, below the thorax, is plainly discern- 

 ible. Lastly, the oral orifice appears, and the 

 general form of the young animal approaches 

 more and more that of the adult. From this 

 a new individual is then in like manner pro- 

 duced by stolon and bud, remaining attached 



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