912 POLYZOA AND TUNICATA 



to the comparative anatomist and the zoologist, this group does not 

 afford much to interest the ordinary microscopist, except in the pecu- 

 liar actions of its respiratory and circulatory apparatus. In common 

 with the composite forms of the group, the solitary Ascidians have 

 a large branchial sac, with fissured walls, resembling that shown in 

 figs. 690, B, and 692 ; into this sac water is admitted by the oral 

 orifice, and a large proportion of it is caused to pass through the 

 fissures, by the agency of the cilia with which they are fringed, into 

 a surrounding chamber, whence it is expelled through the atriopore, 

 or opening of the mantle. This action may be distinctly watched 

 through the external walls in the smaller and more transparent 

 species ; and not even the ciliary action of the tentacles of the Polyzoa 

 affords a more beautiful spectacle. It is peculiarly remarkable in one 

 species that occurs on our own coasts, the Corella parallelogramma, 1 

 in which the wall of the branchial sac is divided into a number of 

 areolse, each of them shaped into a shallow funnel ; and round one 

 of these funnels each branchial fissure makes two or three turns of a 

 spiral. When the cilia of all these spiral fissures are in active move- 

 ment at once, the effect is most singular. Another most remarkable 

 phenomenon presented throughout the group, and well seen in the 

 solitary Ascidian just referred to, is the alternation in the direction 

 of the circulation. The heart, which lies at the bottom of the 

 branchial sac, has its one end connected with the principal trunk 

 leading to the body, and the other with that leading to the branchial 

 sac. At one time it will be seen that the blood flows from the 

 respiratory apparatus to the end of the heart in which its trunk 

 terminates, which then contracts so as to drive it through the sys- 

 temic trunk to the body at large ; but after this course has been main- 

 tained for a time the heart ceases to pulsate for a moment or two, 

 and the course is reversed, the blood flowing into the heart from 

 the body generally, and being propelled to the branchial sac. After 

 this reversed course has continued for some time another pause 

 occurs, and the first course is resumed. The length of time inter- 

 vening between the changes does not seem by any means constant. 

 It is usually stated at from half a minute to two minutes in the com- 

 posite forms ; but in the solitary Corella parallelogramma (a species 

 very common in Lamlash Bay, Arran), the Author has repeatedly 

 observed an interval of from five to fifteen minutes, and in some 

 instances he has seen the circulation go on for half an hour, or even 

 longer, without change always, however, reversing at last. 



The compound Ascidians are very commonly found adherent to 

 seaweeds, zoophytes, and stones between the tide-marks ; and they 

 present objects of great interest to the microscopist, since the small 

 size and transparence of their bodies when they are detached from 

 the mass in which they are imbedded not only enable their structure 

 to be clearly discerned without dissection, but allow many of their 

 living actions to be watched. Of these we have a characteristic 

 example in Amaroucium proliferum, of which the form of the com- 



1 See Alder in Ann. of Nat. Hist. ser. iii. vol. xi. 1863, p. 157; and Hancock in 

 Journ. Linn. Soc. ix. p. 333. 



