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 Pleurohrachia 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 Boluiopsis vitrea has been mistaken for 

 Pleurohrachia. 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 

 ctenophoras 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 Mnemiopsis and Bolinopsis. On 

 the Pacific coast of North America this ctenophore has commonly been 

 called Pleurohrachia bachei. 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 Pleurohrachia. 



In the so-called genus Cydippe, which is probably identical with 

 Pleurohrachia, 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 or 4 protoplasmic processes, 



