644 



the other forms studied, a similar result is attained in a slightly dif- 

 ferent way. 



4) In Siphostoma fuscum. 



The young pipefish when released from the brood-sac of the male 

 is about 8 mm. to 10 mm. in length. The swim -bladder in such a 



specimen consists of 

 an expanded, single- 

 chambered vesicle, 

 at the posterior 



Fig. 10. Diagrams 

 to show the transforma- 

 tions of the pneumatic 

 duct into the posterior 

 chamber of the swim- 

 bladder in Siphostoma 

 fuscum. a swim-bladder 

 at time of release of 

 larva from brood - sac. 

 b shows the stump of 

 the pneumatic duct 

 growing backward to 

 form a U-shaped tube. 

 c shows the pneumatic 

 duct expanded to form 

 the posterior chamber. 

 A anterior end of oeso- 

 phagus, ant.ch anterior 

 chamber, cnt outer con. 

 nective tissue layer. 

 epi epithelium lining 

 the cavity of the swim- 

 bladder, pnd pneumatic 

 duct, part of which 

 atrophies as is shown 

 by .the dotted lines. 

 post.ch posterior cham- 

 ber, sbl cavity of swim- 

 bladder, vpnd vestigial 

 portion of pneumatic duct. 

 xpnd expanding portion 

 of pneumatic duct. 



cnt 



xpnd 



cnt 



vpnd 



end of which enters the long, narrow pneumatic duct which con- 

 nects it with the oesophagus. The oesophageal end of the duct 

 soon atrophies (Fig. 10a). The remainder of the duct soon begins to 

 grow backward in the manner indicated in Fig. 10b, thus forming a 

 curved, U-shaped tube. The limb of the U which is attached to the 

 swim-bladder then greatly enlarges, thus forming the posterior chamber 

 (Fig. 10 c). This latter process takes place in young of about 35 mm. 

 to 40 mm. in length. The other limb of the U-shaped tube remains for 

 some time afterward as a vestige imbedded in the wall of the swim- 

 bladder, but in Siphostoma it disappears in the later stages. 



