TYPE PBOTOCHORDATA. 



607 



a simple circular band, while another runs over the anterior 

 portion in an exceedingly tortuous course. It may be con- 

 sidered as consisting of two portions, one prseoral in position 

 and the other postoral, the two meeting, ' however, at the 

 apex of the body in an ectoder- 

 mal thickening, the apical plate 

 (a), which bears two eyes. 



The mouth {M) opens by a 

 short cBsophagus into a capacious 

 stomach [S), separated by a per- 

 forated partition from the short 

 rectum [R), which opens by the 

 terminal anus. In the anterior 

 portion of the body is a saclike 

 structure {pc), united to the apical 

 plate by a muscular band and 



opening to the exterior by a pore .„„„„„„ 

 / \ -i. i. A Tiii i i.\ ^ ti. , Fig. 277.— ToRNAKiA Larva OP 

 {p) Situated a little to the left of Balamglosms Kowalewskii. 

 the mid-dorsal line. This sac is 

 the proboscis-ccelom, and the pore 

 the proboscis-pore, and in connec- 

 tion with the sac is a smaller sac, 

 the so-called heart, which becomes 

 the proboscis-vesicle of the adult. 

 At a later stage of development 

 two other pairs of ccelomic sacs {cc and tc) make their ap- 

 pearance at the sides of the stomach and give rise to the 

 collar and trunk coelom. The adult form is acquired by the 

 gradual transformation of this larva, there being no sudden 

 metamorphosis. 



The Affinities of the Hemichordata. — There seems little room for doubt 

 but that the Pterobranchia and the Enteropneusta are closely related, 

 so many similar and at the same time peculiar structures being found in 

 both groups. Thus, to mention only some of the more striking features, 

 both groups possess a notochord of a similar character, proboscis-pores, 

 collar-pores and branchial slits, and have the body and coelom divided into 

 three strictly-comparable regions. These remarkable similarities can only 

 be explained on the assumption of a common ancestry for both groups. 



In many respects, too, the Enteropneusta present similarities to the suc- 

 ceeding group, the Oephalochorda, but a discussion of this relationship may 



a = apical plate. 

 ce = collar-coelom. 

 M= mouth. 



p = dorsal pore. 

 pc = proboscis-coelom. 



i? = rectum. 



8 = stomach. 



tc = trunk-coelom. 



