CTENOPHORES OF THE ATLANTIC COAST OF NORTH AMERICA 55 



Order PLATYCTENID^E. 



These are degenerate forms that have largely or wholly lost their 

 organs of locomotion. They are fiat and expanded, the oral-aboral axis 

 being much shortened. They are sessile or creep slowly over sea-weeds. 

 In C(sloplana and Ctenoplana the apical sense-organ is present, but in 

 Tjalfiella of Mortensen it appears to be absent. His specimens were, 

 however, preserved in formalin which may have destroyed the concre- 

 tions. There are two tentacles which are feathered in Ctenoplana and 

 Cceloplana, but simple in Tjalfiella. 



Mortensen's discovery of cydippe-Hke young in the brood-sacs of 

 Tjalfiella proves that these forms are simply degenerate, and specialized, 

 not primitive ctenophores. After being set free in the Cydippe stage the 

 young swim about by means of their combs of cilia but soon settle down 

 and lose these organs of locomotion. 



Tjalfiella tristoma Mortensen. 



Tjalfiella tristoma, Mortensen, 1910, Vidensk. Meddel fra den naturh. Foren. i 

 Kobenhavn, p. 249, Fig. 



This is the most degenerate of the Platyctenidae and is found attached 

 by its oral surface to stems of Unibellula lindahlii at depths from 240 to 

 290 fathoms in Umanak Fjord on the West Coast of Greenland. 



The animal is transparent with yellow tentacle-bulbs, and is about 

 10 to 15 mm. long, flat, laterally compressed with the oral surface 

 attached to the Umbellula, and with an erect, funnel-like tower-shaped 

 projection at each end of the body. A slight projection at the middle 

 of the flattened upper side of the animal may represent the degenerated 

 remnant of an apical sense-organ, but there appear to be no lithocysts. 

 A minute pore opens at this place. There are no combs of cilia. 



The mouth is at the center of the surface of attachment and gains 

 access to the outer world by means of a median ventral furrow in the 

 tentacular plane. This furrow extends upwards along the sides of the 

 body to the two tentacle bulbs which are set within it at the sides of the 

 funnel-shaped tentacle sheaths. The tentacles have no side branches 

 thus differing from those of Ctenoplana, and Cceloplana. The tentacles 

 are provided with colloblasts. There are 4 pairs of knob-like projections 

 on the upper surface of the animal and these mark the position of the 

 sexual organs each consisting of an ovary and a testis which open to 

 the outer world through an ectodermal invagination, as in Ctenoplana. 



These genital cavities contain embryos wliich have ciliated combs 

 and are in the cydippe stage, thus proving that the Platyctenida? are 

 degenerate and not primitive ctenophores. When about to leave the 

 parent animal a deep furrow develops in the tentacular plane along the 

 •oral side of each young ctenophore. The young go through a free- 

 swimming stage before they settle down. 



The canal-system of the adult is branched quite complexly. 



Mortensen's final paper upon this animal will appear in the Report 

 upon the Ctenophora^ of the Danish Ingolf-Expedition, vol. 5, part 2. 



