CTENOPHORA 



153 



broken, whereas the vessels of each lobe having the same name come 

 into communication with each other at their lower ends (Fig. 73 7^, at x), 

 and, on the other hand, each gastral vessel, which up to this time ended 

 blindly, sends out two transverse processes at its oral end, which open 

 into the subtransverse vessels of the same side. With the development 

 of (1) the c»cal pouches (characteristic of Eucharis) above the base of the 

 tentacles (metamorphosed tentacle-sheath) and (2) the dermal papillae 

 the form of the adult animal is reached. 



Chun was able to establish the fact that under certain conditions the 

 Mertensia stage attains sexual maturity, so that the existence of a re- 

 markable heteroyeny is established for the Ctenophora. 



The metamorphosis of the Cestidae, as Chun's observations on Cestus 

 show, proceeds from a Mertensia stage quite similar to that of Eucharis. 

 Here also the sagittal diameter is at first shorter than the transversei 

 although subsequently it so vastly predominates in the ribbon-like body. 

 That which especially characterizes the Cydippidoid early stage of Cestus 

 is the presence of a single swimming plate on each rib, corresponding to 



Fig. 74.— Two stages of development of Cestus veneris (after Chun). A resembles 

 the CydippidEe in form : m, stomach ; mg, gastral vessel, with its processes ; s, 

 eubsagittal t, subtransverse meridional vessel ; B, somewhat older stage with the 

 ciliary plates in their permanent position. 



the uppermost of the four embryonic swimming plates, the lower ones of 

 which become degenerated. The further course of the metamorphosis 

 is tolerably simple. At first the larva is round in cross-section ; then it is 

 flattened in the transverse direction (Fig. 7-i /I), so that the flat ribbon- 

 like shape is more and more expressed. At the same time the short 

 meridional vessels and the gastral vessel grow downward. The latter 

 soon puts forth two transverse processes (Fig. 74 w//), which extend 

 parallel to the lower margin of the larva. Of the meridional vessels the 

 subsagittal (Fig. 74 s) continue to grow out and become arched, while on 

 their upper parts new swimming plates are formed, which at first are 

 placed at right angles to the meridional vessels, but later (corresponding 

 to the conditions of the adult) are placed with their bases in the direction 

 of the meridional vessel (Fig. 74 A and B). The ends of the meridional 

 vessels and of the processes of the gastral vessels come together in 

 the lower.corners of the now trapezoidal, flattened larva (Fig. 74 B) and 



