60 PROCEEDINGS OF THE 



ever, could not be distinctly made out. The second pair were 

 undoubtedly bifurcated — the upper one terminated by two, and 

 the lower by four setse. The third pair lay close to the side, each 

 with one long seta. There were two little setse given oflf from 

 the tail. 



In these instances the ovisac presented the same shape as the 

 ovum of the Amouroucium, near ivhich it was situated. One, 

 however (observed by itself, on the sHde), had the usual pyri- 

 form shape of the entomostracan ovisac, and contained some 200 

 ovules with the red eye-speck; so that probably in the former the 

 pyriform shape was merely concealed by its position in the Amoiir- 

 oaciwn; but whether attached to the wall of the branchial sac at 

 the side near the common orifice, or placed within the substance 

 of the ovary, or in the oviduct, where the young could easily find 

 their way out, could not be clearly ascertained. This entomos- 

 tracan ovisac was found to occur in four of the animals we dis- 

 sected. The species to which it belonged is, of course, unknown ; 

 but its presence in such a situation would seem to show that this 

 little Amouroucium, like its larger relation, has also an attendant 

 entomostracan. 



Ascidia virginea. — In this species the tunic is crystalline, of a 

 clear, transparent texture, through which the internal structure of 

 the animal can be easily seen. The cells of the branchial sac are 

 coloured with brilliant orange-red or scarlet pigment cells, not 

 uniformly laid on, but rather in broken and scattered patches. 

 Just below the stomach-mass the heart is situated, and through 

 the transparent tunic the circulation of the blood is readily seen — 

 more so on one side of the body than the other. 



In most orders of mollusca the blood is propelled by the heart 

 through the arterial vessels, and returns again by a system of veins 

 and sinuses, thus completing a circle. In the tunicata, to which 

 order the ascidians belong, the circulation bears a special interest. 

 In them it is propelled through one set of vessels, and returns 

 again through the same, the course of the circulation being 

 reversed — the same vessels which fulfilled the function of arteries 

 serving also as veins to carry it back to the heart. In none is this 

 observed with such facility as in Ascidia virginea. At intervals the 

 circulation is more rapid, but the time and number of the pulsa- 

 tions seem pretty uniform. After sixty waves have passed, the 

 current gets slower, and gradually ceases about the seventieth. 



