278 TUNICATA. 



chiopoda to the Ascidiae, it presents, at the same time, a 

 beautiful analogy with the elementary forms of the air- 

 breathing organ, as it exists, for example, in the pulmo- 

 niferous Gasteropods." * 



The naked acephalous mollusca (Tunicata, Lamk.) have 

 two, and only two, orifices in their outer tunic, which very 

 often open on the tips of two tubular projections, or papilla?, 

 placed near one another. By one of these, f which is usually 

 the highest, and encircled within its rim with one or two 

 rows of slender tentacular filaments, or furnished with a 

 valve, the water necessary to respiration flows into a large 

 visceral sac, which, while it seems in part to perform the 

 functions of a crop, affords ample space for the display of 

 the aerating blood-vessels. The water, after being breathed, 

 is in general expelled at the same orifice by which it was 

 sucked in ; and, notwithstanding that the observations of 

 several naturalists seemed to prove the contrary, Cuvier was 

 nevertheless inclined to conclude, from his anatomical inves- 

 tigations, that it could not possibly be expelled from the 

 other, which is the vent. The conclusion is only partially 

 correct, for it would be erroneous were we hence led to infer 

 that the animals cannot eject any of the contained water 

 from the vent. The water which distends the body of a 

 healthy submerged ascidian flows into it simultaneously by 

 both orifices in a current so still that no stream is observ- 

 able. Lister was well cognisant of this fact ; and Reaumur 

 states expressly, that the water enters at times, and is driven 

 out by either of the siphonal apertures. J Spallanzani has 

 described the phenomena, in our opinion, more accurately 

 than any other observer.§ Dr. Coldstream says : " It has 



* Owen in Zool. Trans, i. 154.— Also p. 148. 



t In all the compound mollusca, the branchial orifice of the component 

 individuals, tends always to approach to the circumference of the system, as 

 the anus does as invariably to the centre. — Savigni/. 



X Hist, de l'Acad. Roy. des Sciences, 1710, 588. 



§ " In my various excursions on the sea, I have observed that it contains 

 many animals, which, absorbing the water by their mouth, produce a small 

 vertiginous current, that runs into it. The animal in question has not this 

 power. The water enters it almost insensibly, by occupying gradually the 

 internal space which was empty. This I perceived by making use of the 

 lens, but more distinctly by tinging the sea-water with cochineal, as the 

 animal will live several hours in this water without any apparent injury. 

 The red particles of the tincture will then be seen slowly to enter the two 

 apertures with the water, gradually filling the vacuity of the animal without 

 the appearance of a current of any kind. After some time, the slow motion 

 of the particles ceases ; that is, when the internal cavity is completely filled 

 with the coloured water, which I could cause to issue from the apertures at 

 pleasure, by pressing the Ascidia between my fingers. 



