504 POLYZOA AND COMPOUND TCNICATA. 



mach {{), which is situated at the bottom of the sac. Minute 

 animals are often swallowed alive, and have been observed dart- 

 ing about in the cavity for some days, without any apparent 

 injury either to themselves or to the creature which encloses 

 them. In general, however, particles which are unsuited for re- 

 ception into the stomach, are ejected 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 branchial orifice. 



334. The circulation of blood, throughout the entire class, is 

 remarkable for the altej-nation which it presents, from time to 

 time, in the direction of its flow; and this curious phenomenon 

 may be particularly well studied in the PeropJiora. The creep- 

 ing-stalk (Pig. 250) that connects the individuals of any group, 

 contains two distinct canals, which send oft' branches into each 

 peduncle. One of these branches terminates in the heart, which* 

 is nothing more than a contractile dilatation 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 i-espiratory sac, branching off at 

 each of the passages between the oval slits, whilst others are 

 first distributed to the stomach and intestines, and to the soft 

 surface of the mantle. All these I'eunite, and form a trunk, 

 which passes to the peduncle, and constitutes the returning 

 branch. !N"ow whilst at some periods, the heart may be seen 

 vigorously contracting from behind forwards, so as to propel the 

 blood along the course just described, the observer, if he con- 

 tinue to watch, will see its pulsations becoming fainter for a few 

 beats, and the flow in the vessels becoming slower ; and then, 

 after a slight pause, the whole current in all its windings is re- 

 versed. The heart gives the opposite impulse, receiving the 

 blood from the body, and sending it back into the peduncle 

 through the tube that previously conveyed it thence ; while the 

 tube that had previously served to carry the returning stream, 

 now brings the blood from the stem, and distributes it to the 

 branchial sac, mantle, and visceral apparatus, whence it finds 

 its way back to the heart. After the circulation has continued 

 for a certain time in this new direction, the intermission is re- 

 peated; and then a reversal takes place to the first course. The 

 average time during which the circulation persists in each di- 

 rection, seems to be about the same for the one as for the other; 

 but the period between the changes varies as much as from 

 thirty seconds to two minutes. 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. This is the course which it takes in 



