192 IXVRIiTF.l'.lt.VTE ANIMALS. 



ble proportion t>f a sul>.st:ince a|iparently identical with cellahise, 

 which is one of tlie most characteiistic of all vegetable products. 

 The test is lined by a second coat, which is highly muscular, and 

 confers u])on the animal its power of contracting itself and squirting 

 out water. Of the two necks which are placed at the anterior end 

 of a simple Ascidiun (fig. 134, C), one is perforated by the aperture 

 of the mouth, wliilst tlie other sei'ves as an e.xcretory aperture. 

 These two apertures are known respectively as the " oral," or 

 "branchial," ami tlie "atrial" a]icrtures. 



The oral aperture (ii) is usually furnished with a circle of small 

 non-retractile tentacles, and 0])ens into a great i-hand.ier known by 

 various names, but liest as the "respiratory sac" or "branchial .sac." 

 This sac occu[)ies the greater p.art of the cavity of the body (fig. 

 134, A, s), and has its walls perforated by numerous apertures, the 

 sides of which are ciliated. At the bottom of the respiratory sac is 

 a second opening (the iiumfji of some writers) which leads by a short 

 gullet into a capacious stomach {;/). From the stomach an intestine 

 is continued to terminate by a distinct .•luus, which does not com- 

 municate directly with the e.xtcrior, but opens into a .second 

 great chandler, known as the "cloaca" or "atrium" (c). The 

 cloaca, ill turn, opens on the exterior by the second or atrial aperture 

 in the test (n). These two great clianiliers — namely, tlie respiratory 

 sac and the I'loaca — occujiy the greater jiart of the body-cavity, and 

 where their walls come into contact, a free communication is estab- 

 lished between the two by nieaiiM of the ciliated apertures already 

 spoken of as perforating the resjiiratory .sac. The cilia which fringe 

 these apertures all work towards the cloaca, and thus a constant 

 current of water is caused to set in liy the oral aperture, through 

 the res]jiratory sac, into the cloaca, and out again by the atrial 

 aperture. In this way resjiiration is effected, the walls of the les- 

 ]iiratiiry .sac being almost made up of blood-vessels. A distinct 

 iieart is ]iicsciit in all the Tiuiii-nta, but it has a very simple struc- 

 ture. It consists of a siuqile tube, open at both ends, and not pro- 

 viilcd with valves. In cunseiiuence of this, the circulation in the 

 m.ijority of Tunii-arics is |ieriodirally reversed, the blood being 

 driven for a certain number of contractions ill one direction, and 

 then proiiclled for a like jieriod in an o|ipositc direction ; so that 

 " the two ends of the Iieart are alternately arterial and venous." 



The nervous system in the Tiniiru/ii consists of a single ganglion 

 plai-ed on one side of the oral aperture (tig. 134, .A, ii). 



With one or two exceptions all the Tiinirtifit are licrniaphrodite, 

 the (irgaiis of reproduction being situated in a fold of tlie intestine, 

 and upiiiing into the cloaca. The enibiyo is usually at first free, 

 and swims alxnit by means of a long tajl, so that it presents con- 



