668 THE INVERTEBRATA 



contained in an organ known as the glomerulus, which is formed by a 

 puckering of the hinder wall of the proboscis cavity around the end 

 of the notochord. It is thought that this organ acts as a kidnev, taking 

 waste matters from the blood and throwing them into the proboscis 

 cavity, whence they are expelled through the proboscis pores when 

 the organ contracts. From the glomerulus the blood is gathered into 

 two vessels which lead backwards one on each side to a ventral vessel 

 below the gut. From this vessel is supplied a plexus in the wall 

 of the alimentar}- canal, including the bars between the gill openings. 

 From this plexus blood passes to the dorsal vessel. The blood flows 

 forwards in the dorsal vessel and backwards in the ventral. 



The sexes are separate. The gonads are mere sacs lying at the sides 

 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 adult, they are developed from the 

 coelomic wall. 



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

 a pelagic lar\-al stage kno\\Ti as the Tornaria (Fig. 466), which closely 

 resembles the Auricularia lar\-a of holothurians (p. 632), but differs 

 in possessing a perianal band of cilia in addition to the longitudinal 

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

 epithelium which bears the apical tuft of cilia. The larv^a 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. 627, 

 628), is in Balanoglossus the rudiment of the pericardium. In some 

 species the Q^g is larger and there is no Tornaria stage. In all, how- 

 ever, cleavage of the OMjm is complete and gastrulation is bv in- 

 vagination. 



Cephalodiscus and Rhabdopleura (Fig. 467) are minute animals in 

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

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

 upwards. The collar bears in Rhabdopleura 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 

 Rhabdopleura only a gonadial opening on the right side. On the belly 

 is a peduncle which bears buds. In Cephalodiscus these become free; 

 in Rhabdopleura 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 Rhabdopleura has no gill clefts or 

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

 invaginated. 



