DEVELOPMENT OP THE CTENOPHOEA. 107 



The tentacular apparatus, whicn is situated on a level with the 

 funnel, gives origin not only to the main tentacle-stem with its 

 lateral filaments, but also to a small independent stem, which 

 Chun speaks of as a " reserve tentacle." 



The condition of the ribs is very interesting in relation to the 

 changes of form of the larva. By the time that the vascular 

 system has nearly attained its characteristic dichotomous division, 

 the four or five swimming-plates with which each rib had been 

 hitherto provided have become reduced to a single one, to which 

 a nerve runs from the central nervous system. In this change 

 that plate which in each rib lies nearest to the sense-pole has 

 become greatly increased in breadth, while those which follow it 

 become atrophied and finally disappear. 



The 2Ierfensia-\ike flattening of the body in the plane ef the 

 stomach now gradually disappears, and the larva appears circular 

 in transverse section. Soon, however, it presents again a slight 

 flattening ; but this is in the plane of the funnel, and with the 

 growth of the larva becomes more and more decided, the length 

 of the stomach-axis exceeding more and more that of the funnel- 

 axis, until finally the long band-like form, characteristic of the 

 adult Yenus's Girdle, is attained. 



With the band-like extension of the body the " subventral " 

 meridional vessels are drawn out in- the same direction, while 

 the stomach-vessels send off each on the mouth-margin two 

 branches at right angles to the main trunk, and the " subtenta- 

 cular" vessels also diverge more and more to the right and left. 

 At the same time the tentacular apparatus undergoes certain 

 important changes. While the main tentacle still retains its 

 original form, finger-like processes bud forth from the reserve 

 tentacle : those at the proximal part of the tentacle remain fused 

 together ; but at the distal end they are difi"erentiated into separate 

 independent processes. 



Though Chun has not succeeded in observing all the stages of 

 the development of the tentacular apparatus, he regards it as 

 almost certain that it is this reserve tentacle which is ultimately 

 to become the definitive tentacle of Cestum. 



Along the course of the subvcntral vessels new swimming- 

 plates now make their appearance. Though at fii-st arranged 

 nearly vertically, this disposition rapidly yields to a horizontal 

 one as the animal continues to assume its definitive baud-like 

 form. The form of the animal is now nearly that of a rectangle ; 



8* 



