168 THE MICROSCOPE AND ITS REVELATIONS. 



particles, animate or inanimate, the current of water brings, flow into 

 the sac, unless stopped at its entrance by the tentacles (#'), which do not 

 appear fastidious. The particles which are admitted usually lodge some- 

 where on the sides of the sac, and then travel horizontally until they 

 arrive at that part of it down which the current proceeds to the entrance 

 of the stomach (i), which is situated at the bottom of the sac. Minute 

 animals are often swallowed alive, and have been observed darting about 

 in the cavity for some days, without any apparent injury either to them- 

 selves or to the creature which incloses them. In general, however, 

 particles which are unsuited for reception into the stomach are rejected 

 by the sudden contraction of the mantle (or muscular tunic), the vent 

 being at the same time closed, so that they are forced out by a powerful 

 current through the oral orifice. The curious alternation of the circula- 

 tion that is characteristic of the Class generally ( .555), may be particu- 

 larly well studied in Perophora. The creeping-stalk (Fig. 384) that 

 connects the individuals of any group, contains two distinct canals, 

 which send off branches into each peduncle. One of these branches 

 terminates in the heart, which is nothing more than a contractile dilata- 

 tion of the principal trunk; this trunk subdivides into vessels (or rather 

 sinuses, which are mere channels not having proper walls of their own), 

 of which some ramify over the respiratory sac, branching off at each of 

 the passages between the oval slits, whilst others .are first distributed to 

 the stomach and intestine, and to the soft surface of the mantle. All 

 these reunite so as to form a trunk, which passes to the peduncle and con- 

 stitutes the returning branch. Although the circulation in the different 

 bodies is brought into connection by the common stem, yet that of each 

 is independent of the rest, continuing when, the current through its own 

 footstalk is interrupted by a ligature; and the stream which returns from 

 the branchial sac and the viscera is then poured into the posterior part of 

 the heart, instead of entering the peduncle. 



559. The development of the Ascidians, the early stages ot which are 

 observable whilst the ova are still within the cloaca of the parent, pre- 

 sents some phenomena of much interest to the Microscopist. After the 

 ordinary repeated segmentation of the yolk, whereby a ' mulberry mass ' 

 is produced ( 581), a sort of ring is seen encircling its central portion; 

 but this soon shows itself as a tapering tail-like prolongation from one 

 side of the yolk, which gradually becomes more and more detached from 

 it, save at the part from which it springs. Either whilst the egg is still 

 within the cloaca, or soon after it has escaped from the vent, its envelope 

 bursts, and the larva escapes; and in this condition it presents very much 

 the appearance of a tadpole, the tail being straightened out, and pro- 

 pelling the body freely through the water by its lateral strokes. The 

 centre of the body is occupied by a mass of liquid yolk; and this is con- 

 tinued into the interior of three prolongations which extend themselves 

 from the opposite extremity, each terminating in a sort of sucker. After 

 swimming about for some hours with an active wriggling movement, the 

 larva attaches itself to some solid body by means of one of these suckers; 

 if disturbed from its position, it at first swims about as before; but it 

 soon, completely loses its activity, and becomes permanently attached; 

 and important changes manifest themselves in its interior. The pro- 

 longations of the central yolk-substance into the anterior processes and 

 tail are gradually drawn back, so that the whole of it is concentrated into 

 one mass; and the tail, now consisting only of the gelatinous envelope, is 

 either detached entire from the body by the contraction of the connect- 



