28 CTENOPHORES OF THE ATLANTIC COAST OF NORTH AMERICA 



ugal (abaxial) edges lie along a deep groove which extends down the 

 sides of the body adjacent to the oral lappets, from the mouth to the 

 level of the apical sense-organ. The auricle side of this groove is ciliated, 

 while the opposite side is provided with numerous short, simple tentacles 

 which lie immediately over 4 lateral tubes that diverge from the para- 

 gastric vessels. This long, deep furrow at once distinguishes Mnemiopsis 

 from Bolinopsis. Both sides of the long, slit-like mouth are bordered 

 with a row of short, simple tentacles which are continuous with those 

 of the deep, lateral furrows. The central tentacle on each side is longer 

 than the others and gives rise to lateral filaments. The 4 rows of ciliated 

 combs leading to the auricles are only about half as long as the 4 rows 

 extending down the outer surface of the oral lobes. The combs are small, 

 but numerous. The stomodasum is long and narrow and is provided with 

 cilia at its aboral end. The axial funnel-tube is only about one-fifteenth 

 as long as the stomodaeum. The peripheral canals are of fine caliber,, 

 and the meridional ventral canals give rise to simple windings in the 

 oral lappets. The oral lappets are provided with delicate transverse 

 and longitudinal muscle fibers. The lappets are of no appreciable aid 

 in swimming, this being performed almost entirely by the action of the 

 combs of cilia. The creature is almost colorless, being of a slightly milky 

 translucent hue. The ciliated combs sparkle with brilliant iridescence 

 by day and glow with an intense green light when the animal is disturbed 

 at night. The gelatinous substance of this ctenophore is often infested 

 by a pink colored, worm-shaped, parasitic, larval actinian {Edwardsia) . 

 The development (figs. 20-41, plates 6 and 7) has been studied by 

 A. Agassiz. As in other ctenophorae, the animal is hermaphroditic, 

 and the egg is spherical and is enveloped by a thin, structureless mem- 

 brane which is widely separated from the surface of the egg (see fig. 20, 

 plate 6). The deeper portions of the egg are filled with large, slightly, 

 refractive masses of food-yolk, between which there is a delicate reticulum 

 of protoplasm. The superficial layer of the egg is composed of finely 

 granular protoplasm. The segmentation is total but unequal. The 

 first two cleavage planes are meridional and cut in the direction of the 

 longitudinal axis of the future ctenophore from the animal to the vege- 

 tative pole. The third cleavage is also meridional, but shifted to one 

 side and the other, and it divides the embryo into 8 cells, 4 large central 

 and 4 small lateral cells, the 4 small cells being at the extremity of the 

 longer axis of the embryo, which becomes the tentacular axis of the 

 future ctenophore. The 4 small, laterally placed cells then migrate 

 upward toward the animal pole. The next cleavage is equatorial and cuts 

 off 8 small, uppermost cells from 8 large, yolk-laden macromeres. The 

 small cells then begin to divide rapidly and to extend downward, cover- 

 ing over the 8 macromeres, thus beginning to form a gastrula by epibole. 

 Meanwhile the cleavage cavity remains open, so that the cells form a 

 ring surrounding a central space which extends as an opening from the 

 vegetative to the animal pole. The animal pole corresponds to the aboral 

 apex of the future animal. The small micromeres which are to form the 

 ectoderm envelop the macromeres, forming a cap which overlies them 

 (figs. 22 to 24, plate 6). Before the macromeres are inclosed, however, 

 they divide, giving rise to a number of small cells, some of which form. 



