592 THE INVERTEBRATA 



organization of a low grade. Most of them are sessile, and there is no 

 doubt that this habit has established the peculiarities of the group. 



We shall describe the organization and life of the Tunicata by 

 giving an account of a typical example, Ciona intestinalis of the 

 British coasts, one of the simple "ascidians". The adult of this 

 animal (Fig. 443) is a subcylindrical sac, which reaches a height of 

 several inches, sometimes nearly a foot, seated by the blind end upon 

 some solid object on the bottom, and at the other bearing two open- 

 ings, a terminal mouth or branchial opening and an atrial opening, 

 seated on a tubular projection a little way below the mouth. This pro- 

 jection, which marks the dorsal side of the animal, is known as the 

 atrial siphon. Beyond its origin the body narrows as the oral siphon 

 towards the mouth. The latter is surrounded by eight small lobes 

 with red pigment spots between them. The atrial opening has six 

 lobes. Both apertures can be narrowed and virtually closed. When 

 the animal is in water and has them open a current may be seen to 

 set in at the mouth and out at the atrial opening. By sudden contrac- 

 tions of the body water may be forced out of both of them. 



The body is covered by a tough, translucent test, remarkable for 

 being composed largely of tunicin, a substance closely related to 

 cellulose, and therefore not to be expected in an animal. The test is 

 a cuticular secretion of the ectoderm, but contains cells of meso- 

 dermal origin which have wandered into it, and ramifying tubes in 

 which blood circulates, which enter it at a point near the base of the 

 sac. Below the test lies the true body wall or mantle, which contains 

 numerous longitudinal and transverse strands of muscle by which the 

 shape of the body can be altered, and sphincters around the openings, 

 where test and mantle are tucked in for a short distance. 



The alimentary canal (Fig. 444) begins as a tube, the stomo- 

 daeum or buccal cavity, lined by the inturned test. This leads to the 

 very large pharynx, a circlet of tentacles standing at the junction. A 

 short prebranchial zone of the pharynx lies between the tentacle ring 

 and the peripharyngeal band — a couple of ciliated ridges, which run 

 round the pharynx, with a groove between them. In the dorsal 

 middle line of the prebranchial zone stands the dorsal tubercle. This 

 is the protuberant, horseshoe-shaped opening of a ciliated funnel 

 which receives the duct of a subneural gland that lies under the brain. 

 The function of the gland is unknown. The funnel is innervated from 

 the brain and is supposed to be sensory. 



The rest of the pharynx constitutes the spacious branchial chamber. 

 The lateral walls of this chamber consist of a basket work, formed by 

 the subdivision of the original gill clefts. The openings of the basket 

 work (Fig. 445) are known as stigmata. They are longitudinally 

 elongate and stand in transverse (dorsoventral) rows. Between them 



