CTENOPHORA 151 



of the meridional vessels, but a cord of cells wliicli unites 

 the ectoderm to the sexual organs points to their ectodermal 

 origin. Sac-like invaginations of the superficial epithelium 

 have also been observed, which perhaps represent the original 

 fundaments of genital sacs. 



Metamorphosis. — After the proof had been pi'oduced by 

 the observations of J. Price and JoH. Mullek that the young 

 forms of the Ctenophores resemble to a certain extent the 

 adult animals, and that consequently no alternation of 

 generations was interpolated in their life-history, one was 

 inclined to assume for them a direct development. McCkady 

 was the first to show the existence of a rather marked meta- 

 morphosis by his observation that the young Bolin^ just 

 hatching from the egg were formed after the type of the 

 CydippidsB. Since then the details of the metamorphosis 

 have become known through the researches of A. Agassiz, 

 J. W. Fewkes, and especially of C. Chun. 



Since the Gydippidcc, by the absence of anastomoses of the meridional 

 vessels and the bUnd termination of the gastral vessels, retain through- 

 out life the most primitive type of distribution of the vessels, the 

 metamorphosis in them is simple. Nevertheless it should be mentioned 

 that the Pleurobrachise, which are round in cross-section, are compressed 

 in young stages by the shortening of the sagittal diameter, and in this 

 regard recall the Mertensise (Chun). If Chun's hypothesis is confirmed, 

 according to which the remarkable Thoe paradoxa (which is characterized 

 by the possession of a single extensible tentacle projecting from a tentacle- 

 sheath resembling a chimney-i^ot near the sensory body) belongs in the 

 life-history of Lampetia pancerina, then a much more elaborate meta- 

 mori^hosis will have to be ascribed to some of the Cydippidse. 



The metamorphosis of the Lob'itcc has been described by McCrady, A. 

 Agassiz (No. 1, Bolina), Fol (No. 7, Euramphaea), and Fewkes (Nos. 5 

 and 6, Ocyrrhoe, Mnemiopsis), and especially in Chun's (No. 3) extensive 

 presentation of the course of development in Eucharis multicornis. The 

 latter form in particular exhibits a series of larval stages differing from 

 the adult in habit as well as in the course of the vessels. Here again 

 the point of departure is a Mertensia stage having the structure of the 

 Cydippidae (Fig. 63), with distinctly shortened sagittal and elongated 

 transverse diameters, which is all the more striking since in the adult 

 form the opposite condition in the length of the cross-axes exists. In 

 the Ji7-st stage with the fundaments of lobes, which now follows, a con- 

 siderable increase in the length of the meridional vessels is noticeable. 

 At the same time the subsagittal vessels become longer than the sub- 



