UROCHORDA 



143 



They are without head, feet, arms, or shell. Indeed, 

 few animals seem more helpless and apathetic than these 

 apparently shapeless beings. The tubular heart exhibits 

 the curious phenomenon of reversing its action at brief 

 intervals, so that the blood oscillates backward and for- 

 ward in the same vessels. 

 Another peculiarity is the 

 presence of cellulose in the 

 skin. The water is drawn 

 by cilia into a branchial sac, 

 an enlargement of the first 

 part of the intestine, whence 

 it escapes through openings 

 in the sides, to the excurrent 

 orifice, while the particles of 

 food drawn in with the water 

 are retained and passed into 

 the intestine. The larva is 

 active for a few hours, swim- 

 ming by means of a long tail. 

 It looks like a minute tad- 

 pole, and has a notochord and 

 a nervous system closely resembling those of a verte- 

 brate. Afterward it attaches itself by the head, the tail 

 is absorbed, and the nervous system is reduced to a 

 single small ganglion. Thus the animal, whose larval 

 structure is that of a vertebrate (since it possesses a 

 dorsal nerve cord, a notochord in the dorsal region, 

 and gill slits opening to the exterior), degenerates in 

 its adult stage into an invertebrate. 



Besides developing from fertilized eggs, the tunicates 

 also multiply by the process of budding. In Salpa 

 and some other kinds, alternation of generations 

 takes place. All species are marine and some form 

 colonies. 



FIG. 116. Larval stage of a tunicate, 

 showing the notochord, ; the spinal 

 cord, c ', and the sucking disks, d, by 

 which the larva becomes attached 

 previous to changing to the adult 

 condition. Much magnified. 



