TR.U'HYMEnUS.'E — GONIONEMUS. 



345 



The development has been studied by Murbach, 1895, and more thoroughly by Perkins, 

 1003 The eggs are normally laid at about one hour after sunset from about July 1 until the 

 last of September. They are cast out into the water through dehiscence and each female may 

 lay egas every night for a week. The dehiscence of the eggs on any single night is accom- 

 plished however, in a few minutes. The eggs are brown in color and 0.7 mm. in diameter. 

 Segmentation is total and equal; a hollow i-layered blastula results, and then the entoderm 

 is formed by delamination, the cells of the blastula wall dividing at their inner ends and the 

 inner daughter cells forming the entoderm. These entodermal cells soon increase in size and 

 obliterate the blastula cavity and a solid, spindle-shaped, ciliated planula develops. 1 his 

 planula is about I to 1.5 mm. long, 12 hours after the egg was fertilized. Then the ccelentenc 

 cavity appears in the internal entodermal cells near the posterior end of the planula. After 

 swimming through the water, or gliding over the bottom with a wave-like movement, the 

 planula fixes itself to the bottom by its forward end. The mouth breaks through at the upper 

 end of the fixed planula, or hydra as it has now become. Thus the mouth appears at that 

 which was the posterior end in the free-swimming planula. 2 tentacles develop 180 apart, 

 then 2 others in the diameter 90 away from the first pair, thus forming an oral zone of 4 

 tentacles The first tentacles usually appear about one week after the larva has become 

 attached or three weeks after the fertilization of the egg. The mouth becomes cruciform, 

 the arms'of the cross being in the radii of the 4 tentacles; and the coelenteron is also cruciform 

 in cross-section and extended in the planes of the tentacles and mouth-slits. The 4 tentacles 

 become 3 to 4 times as long as the body of the polyp. They often stretch out, touching the 

 bottom, and when the tips of the tentacles touch the bottom they spread out, "forming a sole- 

 like surface which is closely applied to whatever object the polyp is settled upon. Buds 

 develop from the sides of the polyps below the zone of tentacles. These buds are formed from 

 the ectoderm, mesoglcea, and entoderm of the parent polyp. They are at first hernia-like 

 projections from the side of the polyp, but after about 5 days they become elongated, spindle- 

 shaped, and finally they become detached. They occasionally settle down at once upon the 

 bottom, but usually move through the water for 2 to 4 days before this is accomplished. 

 The bud attaches itself by the end which was outermost (;. e., away from the parent), while 

 its mouth develops upon the end which was nearest the parent. The budded larva then 

 acquires 2 and finally 4 tentacles, these developing 3 to 5 days after the larva has become 



nttuciicci 



The development of the medusa from the polyp has not been observed. Perkins surmises 

 that it is probably through direct metamorphosis, as in Lniopr, but of this we have no actual 



proof. . , . , 



Perkins observed some interesting degeneration phenomena in his reared larva, the 



account of which we present in his own words. 



" For some reason or other, not understood at present, the larva in one of my aquarium 

 jars began when three months old to exhibit most singular forms and activities All appear- 

 ance of the hvdra form was lost, ectoderm and entoderm becoming indistinguishable and cell 

 outlines dissolved. The larva in this condition had very much the appearance of an amoeba. 

 The specimens slumped down on the bottom of the aquarium in a shapeless mass, and by pro- 

 toplasmic flowing changed their shape through an endless variety of forms, moving slowly from 

 point to point. Thin pseudopodia were sent out, along which the substance of the organism 

 flowed and by the breaking of the connecting isthmus divided into two. The fragments be- 

 came smaller and smaller until no longer recognizable. These abnormal larva rema.ned alive 

 for six weeks, after which no trace of them was to be seen." 



The youngest medusa found by Perkins had 8 tentacles, the 4 radial being larger than 

 the 4 interradial. Perkins believes that the 4 radially placed tentacles are the 4 tentacles of 

 the hydra-stage. There are 4 lithocysts which are placed in definite positions relative to the 

 g tentacles. 



Thus, to quote from Perkins, if — 



"We look at the bell-margin from the oral side, the newly arisen tentacles in the four quad- 

 rants have apparently crowded in between the sense-organ and the perradial tentacle, which 

 comes before it as the hands of a watch go. The relation which is here exlnbited m the young- 

 est stage of the free-swimming gonosome is the same throughout the growth ot the medusa: 



