32 INTRODUCTION TO CLASSIFICATION. 



pharynx to the body walls, on which the right and left lamellae 

 become continuous, so as to form the lining of the chamber (&), 

 into whicli the second aperture (I) leads, or the " atrial chamber." 

 Posteriorly, or at the opposite end of the atrial chamber to its 

 aperture, its lining membrane (the " atrial tunic ") is reflected to 

 a greater or less extent over the intestine and circulatory organs, 

 sometimes inclosing each of their parts in distinct plications (as 

 in the genus Phallusia), sometimes merely passing over them, 

 and limiting the blood sinus in which they are contained (as in 

 Clavelina, &c.). Where the atrial tunic is reflected over the 

 sides of the pharynx, the two enter into more or less close 

 union, and the surfaces of contact become perforated by larger 

 or smaller, more or less numerous, apertures. Thus the cavity 

 of the pharynx acquires a free communication with that of the 

 atrium; and, as the margins of the pharyngo-atrial apertures 

 are fringed with cilia, working towards the interior of the body, 

 a current is produced, which sets in at the oral aperture, and 

 out by the atrial opening, and may be readily observed in a 

 living Ascidian. 



The Ascidians possess a distinct heart, but of a very simple 

 construction, seeing that it is merely an incomplete muscular 

 tube, open at each end, and devoid of valves. Functionally, it 

 is not less remarkable than structurally ; for, in the great 

 majority of Ascidians, if not in all, it exhibits a regular alter- 

 nation in the order of the peristaltic contractions of its 

 muscular substance, which has no parallel in the Animal 

 Kingdom. The result of this reversal in the direction of the 

 contractions of the heart is a corresponding periodical reversal 

 of the course of the circulation of the blood, so that the two 

 ends of the heart are alternately arterial and venous. 



The perforated pharynx performs the function of a branchial 

 apparatus, the blood contained in its sieve-like walls being 

 subjected to the action of constant currents of aerated water. 

 All Ascidians possess a single nervous ganglion placed upon 

 one side of the oral aperture (&, Fig. 12), and, in all known 

 genera but Appendicularia, it is situated between the oral and 

 atrial apertures, and, indeed, between the oral and anal 

 apertures ; for, in all genera but that mentioned, the intestine, 



