CHORDATA 



265 



varies greatly in appearance in different groups, being sonic- 

 times gelatinous, sometimes leathery, cither transparent or 

 opaque, and with a smooth or warty surface. A typical sea 

 squirt appears externally as a saclike body, attached at one end, 

 with an opening at the opposite end and a second opening at 

 a greater or less distance from the first on the side of the body 

 (Fig. 274). The first of these openings is called the branchial 

 aperture, the second is the atrial aperture. Around these aper- 

 tures there are frequently pigment spots, which are probably 

 sensory and may represent organs of sight. 



The tunic is lined by a portion of the body wall of the animal 

 called the mantle, which is provided with strong muscle bands. 



ritu.s.bd.s 



stiff 



alr.ca.1/ 



or\a,p 



Ir.ap 



iri.bdL 



FIG. 276. Doliolum. Diagram of the sexual form, atr.ap, atrial pore ; atr.cav, atrial 

 cavity; d.lbc, dorsal tubercle; end, endostyle; /it, heart; int, intestine; mus.bds, muscle 

 bands; ne.gn, nerve ganglion; or.ap, oral aperture; ov, ovary; peri.bd, peripharyngeal 

 band ; ph, pharynx ; stig, stigma ; stem, stomach ; test, testis. (After Herdman, from 

 Parker and Haswell's Text-book.) 



The cavity within the mantle is called the atrium ; it corresponds 

 to a portion of the outside world with which it is in direct 

 communication through the atrial aperture. In the atrium the 

 body is freely suspended, being attached only along the ventral 

 side. The branchial aperture or mouth opens into a large 

 chamber, the branchial chamber or anterior portion of the 

 alimentary canal, whose walls are pierced by very numerous 

 gill slits, which thus connect it with the atrium. The branchial 

 chamber opens by means of a short oesophagus into the 

 stomach, and this into the intestine, which terminates on the 

 dorsal side of the branchial chamber by means of an anus 

 opening into the atrium. There is a heart, which possesses the 



