only one-third that of full oceanic salinity. Of more 

 than eighty species, ahnost seventy can be found in 

 warm seas, three live only in arctic and northern 

 waters, three are deep-sea forms. Two species — the 

 sea gooseberry, Pleiirobrachia pileiis, and the flat- 

 tened thimble-like Beroe aictimis — are cosmopoli- 

 tan and found from pole to pole. Distribution varies 

 with temperature changes, and many comb jellies 

 migrate from surface to deeper waters and back 

 with the round of seasons, as Pleiirobrachia pileiis is 

 known to do in the Black Sea. During the cool of 

 spring it feeds at the surface, then descends gradually 

 to about 110 feet as warm weather arrives, and re- 

 mains below until winter weather returns, having 

 followed water temperatures that remained always 

 at about 52 °F. 



In stormy weather comb jellies sink below the sur- 

 face; but their feeble swimming powers are of no 

 avail against wind-whipped waters or strong currents 

 and tides, so that they often accumulate in great 

 swarms. Then they decimate the small animals 

 that float near the surface, including the fry of com- 

 mercially important fishes. Off the New England 

 coast feeding comb jellies are a menace to cod eggs 

 and young. And in England the fisheries experts 

 have good reason to believe that swarms in certain 



Swimming sea gooseberries, Pleiirobrachia pileus. In 

 some the two long tentacles are extended, in others 

 withdrawn. (Endand. D. P. Wilson) 



years wreak such havoc on small herrings as to be 

 one of the important factors in determining the sizes 

 of herring broods in the different years. The transpar- 

 ency of comb jellies readily reveals the truly formi- 

 dable meals they make on copepods, tiny fishes, lar- 

 vae, and eggs. 



In warm seas ctenophores often have a yellowish 

 cast from the yellow-brown, plantlike cells which 

 they harbor. The relationship is apparently like that 

 seen in many tropical protozoans, sponges, and coe- 

 lenterates, which was discussed earlier. 



The biradial symmetry of ctenophores is a two- 

 sided modification of radial symmetry, and the basic 

 body plan reminds us strongly of scyphozoan jelly- 

 fishes. The main cavity of the body is a digestive sac 

 with branches. Though fine particles can be elimi- 

 nated through two small pores near the upper pole, 

 the main opening of the digestive sac is still the 

 mouth, which serves also for ejecting fish heads and 

 other sizable wastes. Between the fragile digestive 

 lining and the delicate outer covering is a great bulk 

 of secreted jelly which lends firmness and buoyancy, 

 and in the jelly are various cells including long mus- 

 cle fibers. Comb jellies do not pulsate like jellyfishes, 

 but the elongated forms swim by gentle undulations 

 of the whole body, and the aberrant flattened ones 

 can creep about. When dropping vertically or ac- 

 tively moving along, the mouth end precedes. Rest- 

 ing or feeding comb jellies, however, often hang from 

 the surface with the mouth up. 



Both ovaries and testes occur in all individuals; 

 they can usually be seen through the transparent 

 body as they hang from the walls of the eight diges- 

 tive canals that run below the rows of combs. The 

 eggs and sperms are shed through the mouth, and 

 the egg, fertilized in the open water, develops into a 

 little free-swimming larva, the cydippid, which looks 

 like a miniature of a sea gooseberry, with combs and 

 two long tentacles. In the more highly developed 

 ctenophores the cydippid must undergo reduction or 

 loss of the tentacles, besides many other changes, be- 

 fore it achieves the adult form. 



The ctenophores are usually divided into two 

 classes, one with tentacles and one without. 



Comb Jellies with 

 Tentacles 



{Class Tentaculata) 



THE CYDIPPIDS 



The comb jellies that have departed least from the 

 primitive ctenophore stock are the cydippids, the lit- 

 tle globular, egg-shaped, or pear-shaped relatives of 

 the sea gooseberry. Pleiirobrachia pileus. referred to 

 earlier as having a world-wide distribution, has an 



