AND PHYSIOLOGY OF THE TUNICATA. 327 



the visceral mass from the right to the left side. In A. intesti- 

 nalis, ia which there is developed an abdominal chamber, it is 

 doubled upon itself, and lies in this chamber towards the dorsal mar- 

 gin and between the stomach and the bottom of the branchial sac. 

 The heart in Siycla is very long, and narrower than usual ; in this 

 form it lies between the inner tunic and lining membrane on the 

 left, and a little way from the posterior extremity of the mantle, 

 following the curvature of, but at some little distance from, the ali- 

 mentary tube. The posterior extremity opens into the dorsal 

 branchial channel a considerable way up the endostyle ; the ven- 

 tral extremity is attached to the stomach, to either side of which 

 it gives a branch in the usual manner. In JPelonaia the heart is 

 likewise in connexion with the left side of the mantle, and in other 

 respects resembles the arrangement in Stycla. And in Molgula 

 it holds much the same situation — but is placed between the re- 

 productive mass which is above it, and a hollow cylindrical body 

 with hard walls, the nature of which is not understood. 



The branchial sac is usually more complicated than is generally 

 supposed. Hitherto its mechanism has been spoken of in this 

 communication ordy so far as was necessary to the full compre- 

 hension of the blood-system ; it is now time to say something re- 

 specting its more minute structure. In all the Tunicates there 

 must of necessity be present the two great branchial or thoracic 

 channels (the dorsal in connexion with the endostyle, and the 

 ventral at the opposite side of the thorax), even when the bran- 

 chial sac is only partially or not at all developed ; and in every 

 instance where a true gill is present, the transverse channels or 

 primary vessels must also exist. These latter may be considered 

 the essential or elementary parts of the respiratory organ ; the 

 minute details, consisting of secondary vessels, are variable, even 

 in very closely allied species, and are not always present. 



The simplest form of the organ that occurs in the genus 

 Aseidia is found in A. venosa. In this species the transverse or 

 primary vessels, or channels, are placed at regular intervals, and 

 scarcely vary at all in size ; and between and opening into them 

 at right angles are numerous smajl, longitudinal, secondary vessels 

 divided by elongated spaces or stigmata ; so that the whole forms 

 a reticulation of vessels, in which the transverse channels are large 

 and distant, the longitudinal ones small and numerous and di- 

 vided only by narrow open spaces. Or the structure may be de- 

 scribed, for convenience, as it frequently is, as a vascular mem- 

 brane with large transverse channels and minute longitudinal 



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