406 W. K. BROOKS OX THE LIFE-HISTORY 



I. JEginopsis — Egg = Planula = Actinula = Medusa X <C Eggs. 



As the floating- hydra stage of Tubularia is well known under the familiar name Acti- 

 nula and as il seems desirable to use a special term for the free hydra stage of medusae 

 as distinguished from a sessile hydroid, I shall employ this word for this purpose, desig- 

 nating by it a free or floating hydra which may or may not be ciliated. 



I have shown that we have in Liriope and its allies a life-history which i< very similar 

 to that of ./Eginopsis, with numerous secondary modifications, most of which are due to 

 the fact that the gelatinous substance of the umbrella begins to be secreted between the 

 endoderm and the ectoderm at a very early stage in the life of the embryo. The accel- 

 eration of the formation of the umbrella is exactly paralleled by innumerable similar 

 phenomena in the lives of nearly all of the higher metazoa, and it therefore presents no 

 difficulties ; and if we imagine the gelatinous substance absent, the mouthless, untentac- 

 ulated, ciliated Liriope larva, shown in PI. 41, fig. 3, is obviously a planula with an outer 

 layer of ectoderm and a central capsule of endoderm. It has a spacious digestive cav- 

 ity; the two layers are separated by a gelatinous substance; and in our species, the cilia 

 are restricted to a small part of the outer surface: but, in spite of these secondary modi- 

 fications, it is clearly a planula. It soon acquires a mouth and four solid tentacles, and 

 becomes converted into the floating hydra or actinula, shown in PI. 41, fig. 8, with ecto- 

 derm, endoderm, stomach, mouth, lasso-cells, and four tentacles, but with neither sub- 

 umbrella, sense organs nor veil. This larva becomes converted into an adult medusa by 

 the growth of the tentacular zone into an umbrella, and by the acquisition of sense organs, 

 precisely like the ^Eginopsis larva, and as each egg gives rise to only one adult the life- 

 history is simple and direct, with a planula stage, a hydra stage and a final medusa stage, 

 and it may therefore be represented by the same diagram as that which was used for 

 ^Eginopsis. 



II. Liriope — Egg r= Planula = Actinula = Medusa X <^ Eggs. 



In our common American Xareomedusa, Gunina odonaria, the fact that the larva is 

 a true hydra was long ago pointed out by McCrady. The planula stage of this species 

 has never been observed, but the resemblance between the ciliated, bitentaculated hydra 

 shown in our PI. 43, fig. 1 and Metschnikoff's account of the .zEginopsis larva at the 

 same stage is so close that we have every reason for believing that, in this species also, 

 the hydra stage is preceded by a planula stage without a mouth or tentacles. The hydra 

 soon acquires two more tentacles and is then fundamentally like the four-tentaeled hy- 

 dra of Liriope. The number of tentacles soon increases to eight, and as is shown in our 

 figs. 3,8 and 4, the hydra becomes converted into a medusa by the outgrowth of the ten- 

 tacular zone and the acquisition of sense organs. Solar, the life-history of our Cunina is 

 as simple as that of JEginopsis or Liriope, but it is complicated by the occurrence of 

 a sexual multiplication in the larva and also by parasitism. The actinula, or floating cili- 

 ated hydra, after gaining access to the sub-umbrella of a Turritopsis, gives rise to buds 

 from the aboral end of its body, behind the circlet of tentacles; each of these buds is a 



