106 PBOr. ALLMAN ON THE 



physiological role of the blind sacs, no trace of which has been 

 found in any of the other lobed Ctenophora *. 



Among the most valuable observations of Chun are those which 

 he has made on the development of Gestum, and which, with the 

 exception of some fragmentary observations of Kowalewsky, 

 afford almost the only information we possess regarding the 

 embryology of this most interesting and highly aberrant type of 

 Ctenophora. Chun has succeeded not only in observing the just 

 hatched larva of Cestim veneris, but in following almost con- 

 tinuously its metamox'phosis. 



Gestum, when just hatched, presents the Mertensia type of 

 form, and is almost identical in shape with the young Eucharis- 

 larva, the difference between the two being found mainly in the 

 tentacles, each of whose lateral filaments carry in the Gestum- 

 larva a terminal knob of peculiar prehensile cells (Grreifzellen). 

 Each rib carries from four to five swimining-plates. 



The Merfensia-like compression of the body is very obvious, 

 the diameter in the plane of the funnel considerably exceeding 

 that in the plane of the stomach ; and this is rendered all the 

 more significant when we bear in mind that in the adult not 

 only is the compression of the body the reverse of this, but the 

 length of the stomach-axis in the great band-shaped Yenus's 

 Grirdle may be even a hundred times greater than that of the 

 funnel-axis. Chun here remarks, with justice, that in few groups 

 of the animal kingdom does the post-embryonal metamorphosis so 

 strikingly recapitulate, even in the details of organization, the 

 adult forms of more simply organized groups, as do the larval 

 states of Gestum and the lobed Ctenophora recapitulate the 

 configuration of the adult Mertensice. 



The gastrovascular system shows, as yet, no obvious difference 

 between central and peripheral vessel-trunks ; but as the larva 

 increases in size we find the vessels rapidly assuming their de- 

 finitive distribution, and presenting a dichotomy so regular and 

 normal that Chun has taken the Cfesi^wm-larva as an illustrative 

 example of the typical form of the distribution of the vessels in 

 the Ctenophora. 



* We cannot avoid being here reminded of very similar sacs in certain 

 Polyzoa, as well as of the segmental organs of the Annelida. It would be 

 rash, however, to push the comparison further, and insist on a homological 

 relation between the blind sacs of Eucharis and the segmental organs of the 

 Annelida. 



