248 



THE PROTOCHORDATA. 



wall of the proboscis known as the proboscis-pore. In 

 B. Kowalevskii this pore lies asymmetrically to the left of 

 the dorsal middle line (Fig. 115), while in B. Kupjferi a 

 corresponding opening occurs to the right of the middle 



line, so that in this species 

 there are two proboscis- 

 pores constituting a sym- 

 metrical pair. 



The left proboscis-pore 

 of Balanoglossus is obvi- 

 ously to be compared with 

 "i-fJ*-* the praeoral pit of Amphi- 



oxus. 



The collar-cavities also 

 open to the exterior by 

 pores, one on each side 

 underneath the dorsal pos- 

 terior free fold of the 

 collar, and on a level with 

 the opening of the first 

 gill-slit. These are the 

 funnel-shaped collarporcs. 

 Spengel states that water 

 is taken in through the 

 collar-pores into the cavity 

 of the collar in order to swell the latter up, so that it 

 may serve as an accessory organ of locomotion in so far 

 as an alternate inflation and collapse of the collar would 

 assist the animal in its slow burrowings in the sand. 



Fig. 117. — Diagrammatic horizontal 

 section througii an embryo of Balanoglos- 

 sus (type of the direct development), to 

 show the origin of the body-cavities as 

 archenteric pouches. (After Bateson.) 



ap. Tuft of cilia at the apical pole 

 (indication of an apical plate), bc^. Probos- 

 cis-cavity, bc^. Collar-cavities, cij^. Trunk- 

 cavities, cb. Circular band of cilia. 



