CTENOPHORES OF THE ATLANTIC COAST OF NORTH AMERICA 13 
later these rows separate, forming 8 rows, 45° apart. The tentacles 
make their appearance very early in development, indeed almost as 
early as the combs of cilia. At first the tentacles are mere knob-like 
protuberances, but they rapidly elongate into simple, slender threads, 
and later the lateral branches make their appearance. These lateral 
branches begin to develop near the proximal end of the tentacle. At 
first the tentacle bulb arises from the outer surface of the side of the 
body, but later it becomes overgrown, so that finally it comes to lie at 
the bottom of a deep sheath-like cleft. The body of the young Pleuro- 
brachia becomes pear-shaped and elongated, the polar axis being about 
1.5 times as long as the tentacular axis. Later these axes come to be 
almost equal, each to each, and the body is nearly spherical. The pro- 
cess of development of the central part of the gastro-vascular system 
is similar to that in Mnemiopsis. The young Pleurobrachia remains 
within the egg envelope until it has become pear-shaped and has 2 long, 
simple tentacles. While within the egg it swims about and rotates upon 
its axis, very much as does the adult animal in the water. When float- 
ing passively in calm water the adult ctenophore remains with its mouth 
upward. When it swims it progresses with the mouth forward. 
This species is an inhabitant of the Arctic, and according to Moser, 
1908, 1909, also of the Antarctic and Pacific oceans. It extends south- 
ward in Europe into the North Sea, and in America along the coast of 
New England as far as Cape Cod, although winter storms drive it far 
southward along our coast. It is recorded from the Bermudas, but 
here I think the young of Bolinopsis vitrea has been mistaken for 
Pleurobrachta. Early in April it is found on the southern coast of New 
England (Newport Harbor), but as the water becomes warmer with the 
advance of spring it disappears and is not seen through the summer 
months. In the cold waters of the coast of Nova Scotia, or Maine, it 
frequently forms vast swarms during the summer, and at times these 
ctenophore almost touch one another over wide areas. In common with 
other ctenophore it approaches the surface most readily on calm days 
and a moderate ripple is usually sufficient to cause it to sink. This 
species appears, however, to be not so sensitive to disturbance of the 
water as are the larger forms, such as Muemiopsis and Bolinopsis. On 
the Pacific coast of North America this ctenophore has commonly been 
called Pleurobrachia bachet. According to Moser it is found off New 
Zealand and in the South Seas, but she studied only preserved material, 
an almost hopeless task with ctenophores. I have never seen it at 
Tortugas, Florida. 
Curreri, 1900, records it from Messina, Mediterranean, stating that 
he found it there in great numbers in April, 1897; it seems possible, 
however, that he may have seen the young of Bolinopsis, which closely 
resembles the adult of Pleurobrachia. 
In the so-called genus Cydippe, which is probably identical with 
Pleurobrachia, Bethe, 1895 (Biol. Centralblatt, Bd. 15, p. 140), believes 
that he has demonstrated by means of methylen blue that there is a sub- 
epithelial nerve-plexus, the ganglion cells being multipolar, each with 
3 0r 4 protoplasmic processes 
9 
