POLYZOA AND TUNICATA. 



165 



reversed course has continued for some time, another pause occurs, and 

 the first course is resumed. The length of time intervening between the 

 changes does not seem by any means constant. It is usually stated at from 

 half-a-minute to two minutes in the composite forms; but in the solitary 

 Ascidia 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 FIG. 382. 



seen the circulation 

 go-on for half-an-hour, 

 or even longer, without 

 change, always, how- 

 ever, reversing at last. 



556. The Compound 

 Ascidians are very 

 commonly found ad- 

 herent to Sea- weeds, 

 Zoophytes, and stones 

 between the t i d e- 

 marks; and they pre- 

 sent objects of great 

 interest to the Micro- 

 scopist, since the small 

 size and transparence 

 of their bodies when 

 they are detached from 

 the mass in which they 

 are imbedded, not only 

 enables their structure 

 to be clearly discerned 

 without dissection, but 

 allows many of their 

 living actions to be 



watched. Of these we have a characteristic example 

 in Amoroucium proliferum; of which the form of 

 the composite mass and the anatomy of a single 

 individual are displayed in Fig. 382. Its clusters 

 appear almost completely inanimate, exhibiting no 

 very obvious movements when irritated; but if they 

 be placed when fresh in sea-water, a slight pouting 

 of the orifices will soon be perceptible, and a constant 

 and energetic series of currents will be found to 

 enter by one set and to be ejected by the other, 

 indicating that all the machinery of active life is 

 going-on within these apathetic bodies. In the tribe 

 of Polyclinians to which this genus belongs, the body 

 is elongated, and may be divided into three regions, 

 the thorax (A) which is chiefly occupied by the respiratory sac, the abdo- 

 men (B) which contains the digestive apparatus, and the post-abdomen 

 (c) in which the heart and generative organs are lodged. At the summit 

 of the thorax is seen the oral orifice c, which leads to the branchial sac c\ 

 this is perforated by an immense number of slits, which allow part of the 

 water to pass into the space between the branchial sac and the muscular 

 mantle, where it is especially collected in the thoracic sinus/. At k is 



Compound mass of Amoroucium, 

 proliferum with the anatomy of a 

 single zooid: A, thorax; B, abdomen; 

 c, post-abdomen; c, oral orifice; e, 

 branchial sac; /, thoracic sinus: i, 

 anal orifice ; i', projection overhanging 

 it; j, nervous ganglion; fc, oeosphagus; 

 t, stomach surrounded by biliary tu- 

 buli; m, intestine; n, termination of 

 intestine in cloaca; o, heart; o', peri- 

 cardium; p, ovarium; p', egg ready to 

 escape; q, testis; r, spermatic canal; 

 r', termination. 



cloaca. 



of this canal in the 



