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Show 32 ON CLASSIFICATION. as the margins of the pl1aryngo-atrial apertures are fringed with cilia, working towards the interior of the body, a current is produced, which sets in at the. orv.l apertl~re, ~n~ out ~Y. tho atrial opening, and may be readily observed In a living AscJ<han. The Ascidians possess a distinct heart, but of a very siinple construction, seeing that it is Inorely an inco1nplete 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 tho order of the peristaltic contractions of its muscular substance, which has no parallel in the Animal IGngdom. 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 ono side of the oral aperture (l, 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 apertures ; for, in all genera but tiwt mentioned, the intestine, after it has made its hoornal bend, curves down 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 upou its surface, not a calcareous shell, but a case or "test," which may vary in consistence from jelly to har~ leather or horn. Aud it is not one of the least remarkable characteristics of the group that this test is rendered solid, by impregnation with a substunce identical in all respects with that " cellulose" wbir.h is tho 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 circumstance was justly regarded as one of the n1ost rmnarkaLlo factl:l of c01nparative physiology. 'l'HE LAMEJJLIBHANCHTA'l'A. 38 Tho last common and distinctive peculiarity of the Ascidians which I have to mention, is one which acquires importance only from its constancy. The middle of the hromal wall of th pharynx, from near the oral to the cesophageal end, in all these ani1nals, is pushed out into a longitudinal fold, the bottom of which projects into a blood sinus, and has a rnuch thickened epithelial lining. Viewed from one ~ide, the bottom of the fold consequently appears like a hollow rod, and has been termed the "endostyle" (m, Fig. 11). The functions of this structure are unknown, but it has been noticed in all genera of Ascidians hitherto examined. In the next group, that of the LAMELLIBRANCHIATA, which comprises those creatures which we know as mussels, cockles, and scollops, and all the fabricators of what are commonly known as bivalve shells (except the Brachiopoda), we meet with Fig. 12. Fig. 12.-Sect :onal diagram of a f1 cshwater mussel (Anodon). A A, mantle, th,~ right lobe of which is cut away; JJ, foot; C, branchial chamber of the mantle cavity; D, anal chamber ; I, anterior adductor muscle ; I I, posterior adductor muscl e ; a, mouth; b, stomach; c, intestine, t.he turns of which are supposed to be seen tl~rough the side walls of the foot; d, r <>ctum; e, anus ; f, Yentricle; g, am·icle ; h, gills, except i, right external gill, largely cut away and tumed back; ll, labial palpi ; l, (.:ere?ral ; m, pedal ; n, pari eto-splanehnic ganglia ; o, apcrt1n·e of the organ of BoJanus ; p, pericardium. an important advance in organization. In all these animals, the body} as is exemplified by the diagrmn (Fig. 12) of a fl'eslnvatcr D |
