32 ON CLASSIFICATION. 



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 maybe 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 alternation 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. 11), and, in all known genera 

 but Appendicularia, it is situated between the oral and atrial 

 apertures, and, indeed, between the oral and anal aj)ertures ; 

 for, in all genera but that mentioned, the intestine, after it has 

 made its haemal bend, curves clown towards the neural side of the 

 body, and opens into the atrium on that side of the body, and 

 behind the nervous ganglion. 



The outer integument of the Ascidians secretes upon its 

 surface, not a calcareous shell, but a case or " test," which may 

 vary in consistence from jelly to hard leather or horn. And it 

 is not one of the least remarkable characteristics of the group 

 that this test is rendered solid, by impregnation with a substance 

 identical in all respects with that " cellulose ' : which is the 

 proximate principle of woody fibre, and forms the chief part of 

 the skeleton of plants. Before the discoveries of late years had 

 made us familiar with the production of vegetable proximate 

 principles by the metamorphosis of animal tissues, this circum- 

 stance was justly regarded as one of the most remarkable facts 

 of comparative physiology. 



