192 INVERTEBRATE ANIMALS. 



ble proportion of a substance apparently identical with cellulose, 

 ■which is one of the most characteristic of all vegetable products. 

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

 confers upon the animal its power of contracting itself and squirting 

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

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

 of the mouth, whilst the other serves as an excretory aperture. 

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

 " branchial," and the " atrial " apertures. 



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

 non-retractile tentacles, and opens into a great chamber known by 

 vai'ious names, but beat as the " respiratory sac " or " branchial sac." 

 This sac occupies the greater part 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 mouth of some writers) which leads by a short 

 gullet into a capacious stomach (g). From the stomach an intestine 

 is continued to terminate by a distinct anus, which does not com- 

 nmnicate directly with the exterior, but opens into a second 

 great chamber, known as the "cloaca" or "atrium" (o). The 

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

 in the test (a). These two great chambers — namely, the respiratory 

 sac and the cloaca — occupy the greater part of the body-cavity, and 

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

 lished between the two by means of the ciliated apertures already 

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

 these ajjertures all woik towards the cloaca, and thus a constant 

 ciirrent of water is caused to set in by tlie oral aperture, through 

 the respiratory sac, into the cloaca, and out again by the atrial 

 aperture. In this way respiration is effected, the walls of the res- 

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

 heart is present in all the Tunicnta, but it has a very simple struc- 

 ture. It consists of a simple tube, open at both end.s, and not pro- 

 vided with valves. In consequence of this, the circulation in the 

 majority of Tunicaries is jjeriodically reversed, the blood being 

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

 then propelled for a like period in an opposite direction; so that 

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



The nervous system in the Timivata consists of a single ganglion 

 placed on one side of the oral aperture (fig. 134, A, 7i). 



With one or two exceptions all the Tunicata are hermaphrodite, 

 the organs of reproduction being situated in a fold of the intestine, 

 and opening into the cloaca. The embryo is usually at first free, 

 and swims about by means of a long tail, so that it presents con- 



