590 THE INVERTEBRATA 



in the anterior region of the trunk. When they are ripe, openings 

 break through from them to the exterior. Though they have no con- 

 nection with the coelom of the aduh, they are developed from the 

 coelomic wall. 



In most species the egg is small, and development passes through 

 a pelagic larval stage known as the Tornaria (Fig. 441), which 

 closely resembles the Auricularia larva of holothurians (p. 556), but 

 differs in possessing a perianal band of cilia in addition to the longi- 

 tudinal band, and in the presence of a couple of eyespots in the 

 patch of epithelium which bears the apical tuft of cilia. The larva 

 presently sinks to the bottom and undergoes a gradual transformation 

 into the adult, retaining its original symmetry. The pulsating vesicle, 

 which in echinoderms becomes the madreporic vesicle of the adult 

 (pp. 551, 552), is in Balanoglossus the rudiment of the pericardium. 

 In some species the Qgg is larger and there is no Tornaria stage. In 

 all, however, cleavage of the ovum is complete and gastrulation is 

 by invagination. 



Cephalodiscus and Rhahdopleura (Fig. 442) are minute animals in 

 which, owing to a protrusion of the ventral surface, the body is vase- 

 shaped and the gut drawn into a U, so that the anus opens upwards. 

 The collar bears in Rhahdopleura two and in Cephalodiscus several, 

 hollow, branched arms which by means of cilia collect the food of the 

 animal. On the forepart of the trunk are in Cephalodiscus the single 

 pair of gill clefts and the pair of gonadial openings, in Rhahdopleura 

 only a gonadial opening on the right side. On the belly is a peduncle 

 which bears buds. In Cephalodiscus these become free ; in Rhahdo- 

 pleura they remain in continuity with the parent so that a colony of 

 zooids is formed. Both genera have all the characteristic features of 

 Balanoglossus, save that Rhahdopleura has no gill clefts or glomerulus, 

 and that in both the dorsal nerve patch of the collar is not in- 

 vaginated. 



SUBPHYLUM TUNIC AT A (UROCHORDA) 



Chordata without coelom, segmentation, or bony tissue; with a 

 dorsal atrium in the adult; notochord restricted to the tail, which is 

 present in the larval organization only ; the central nervous system 

 removed from the surface of the body and in the adult degenerate ; and 

 a test, usually largely composed of a substance (tunicin) related to 

 cellulose. 



In their adult form, the members of this group are extraordinarily 

 unlike the rest of the phylum. They have lost all the characteristic 

 features of chordate animals except the gill clefts, and are rather 

 shapeless objects which lead a sluggish existence by means of an 



