LiNGULin^i:. 



-MOLLUSCA. TuNicATA. 



381 



Family— LINGULID^. 



The Liugulas (represented on Plate 11, fig. 27, by 

 Linjula anatiiia) have elongated, fleshy oral arms, 

 fringed externally with numerous cirrhi, and situated 

 on each side of the mouth. The valves of the shell 

 of Liitgula are thin, depressed, nearly equal, without a 



hinge, but held together by muscles, and supported by 

 a thick peduncle, which comes out between the beaks. 

 The shell is almost flexible, and is covered with a hard 

 olive periostraca. There are seven species known, all 

 natives of the seas of warm climates, where they are 

 found perforating the mud of shallow baj's, or dwelling 

 in mud or sandy mud in some of the harbours. 



Class VI.— TUNICATA (The Tunicaries). 



" The lowest order of Acephalous Mollusca," says 

 Jlr. Woodward, " are called Tunicaries, being protected 

 by an elastic tunic in place of a shell. They are ex- 

 tremely unlike shell-fish in appearance, and are denied 

 a place in most works of conchology ; having no hard 

 skeleton, they neither furnish objects for the cabinet of 

 the collector, nor materials for the speculation of the 

 geologist." A history of molluscous animals, however, 

 would be imperfect without an account of these " shell- 

 less mollusks," as Cuvier calls them. The Tunicala 

 or Tunicaries then, may be characterized as molluscous 

 animals which have no true shell, but are enveloped 

 in a soft, organized, coriaceous, or gelatinous tunic or 

 mantle. This tunic is constructed in the form of a 

 sac with two openings, or shaped like a tube, of greater 

 or less dimensions, open at both ends ; and is adherent 

 by its base, or by a long flexible peduncle, to some 

 foreign body, as stones, sea-weeds, &c., or else floats 

 freely on the surface of the water. Inside this sac we 

 find a second tunic, which is extremely muscular and 

 always smooth, while the outer surface is sometimes 

 rough and warty. Within this tunic are lodged the 

 viscera, consisting of well-defined organs of respiration, 

 •circulation, and digestion, and a nervous system. The 



Ascidiau, laid upeD to show brancliial sac. 



respiratory or branchial organ is in form of a sac of an 

 oblong, oval, or rectangular shape, and in the Ascidians 

 is a large bag of vascular network furnished with vibratile 

 cilia, and being jjorous, allowing the water to pass 

 readily through it into the mantle cavity, and thence 



out by the exhalant orifice — (fig. 231). "The heart is a 

 simple, elongated, vasiform muscle, inclosed in a peri- 

 cardium, attached to the branchial sac, continued at 

 either end into a vessel — the ramifications of one being 

 expended chiefly upon the respiratory oi'gan, those of the 

 otherupon thevisccra and tunics of thebody. According 

 to the direction of the circulatory currents the one will 

 be an artery, the other a vein, and the circulation itself 

 will be pulmonic and systematic." — (Owen.) Some of 

 the Compound ascidians have the tunic so transparent, 

 that the circulation can be seen going on within. " A 

 very singular condition of the circulatory system has 

 thus been detected. The blood actually moves back- 

 wards and forwards to and from the heart in the same 

 vessels, as it was supposed to ebb and flow in the 

 human veins before Harvey's great discovery." — • 

 {Owen.) The digestive organ consists of a stomach ; 

 an oblong, dilated sac ; an intestine, disposed in a 

 sigmoid flexure ; and a simple follicular liver. The 

 nervous system consists of a ganglion, from which can 

 be traced certain filaments diverging to each a])erture 

 of the sac, and others to supply the respiratory sac. 

 The two orifices or openings mentioned above as ex- 

 isting in the sac or tunic, are branchial and anal. 

 Through the upper or in-current, as it is called (the 

 branchial orifice), the water enters, caiTying with it the 

 nutrient particles floating in the water ; these molecules 

 are introduced by the action of immerous microscopic 

 cilia through this orifice into the esophagus, and from 

 that to the stomach. The alimentary excretions and 

 the generative products are expelled through the lower, 

 ex-current, or anal aperture, by the contraction of the 

 muscular tunic. This is so powerful as to enable the 

 animals to squirt the water out in a sudden jet. " The 

 only vital actions," says Professor Owen, " obvious to 

 ordinary vision, are an occasional ejection of water 

 from the orifices of the tunic by a sudden contraction 

 succeeded by a slow ami gradual expansion. Such 

 contractions and expansions, aided by the ciliary cur- 

 rents which the microscope has detected, and the peri- 

 staltic movements of the alimentary, circulating, and 

 secerning tubes, are all the actions which the organic 

 machinery has to perform in the living Ascidian." A 

 curious discovery was made some years ago in the 

 composition of the tunic of these animals. Dr. Schmidt 

 ascertained that it contained a large proportion of cel- 

 lulose, a ternary organic substance previously believed 

 to be peculiar to vegetables. The sexes are united in 

 almost all the Ttmicata; and the young produced from 

 eggs undergo a regular series of changes or meta- 



