CTENOPHORES OF THE ATLANTIC COAST OF NORTH AMERICA 55 
Order PLATYCTENIDZ. 
These are degenerate forms that have largely or wholly lost their 
organs of locomotion. They are flat and expanded, the oral-aboral axis 
being much shortened. They are sessile or creep slowly over sea-weeds. 
In Celoplana 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 
Celoplana, but simple in Tjalfiella. 
Mortensen’s discovery of cydippe-like 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 Platyctenide and is found attached 
by its oral surface to stems of Umbellula 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 
to 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 Celoplana. 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 which have ciliated combs 
and are in the cydippe stage, thus proving that the Platyctenide 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 Ctenophore of the Danish Ingolf-Expedition, vol. 5, part 2. 
