36G W. K. BROOKS ON THE LIFE-HISTORY 



gains access to the Carmarina, never becomes converted into a perfect medusa, but 

 remains as a degraded nurse, from which other larva are budded, and as Uljanin points 

 out, we have in this case a true alternation of generations. 



The Evolution of oun Knowledge of the Life-History of the Xakcomedus.i:. 

 The growth of our knowledge of the Xarcomedusa? forms one of the most remark- 

 able chapters in the history of zoology, and I shall review it at some length, in order 

 to exhibit the life-history of our American Cunina octonaria in its true relations, and 

 also to show by what slight increments our knowledge has grown. The life of an ani- 

 mal which passes part of its time inside the body of another as a parasite, and then, 

 assuming quite a different form swims at large in the water, presents a very perplexing 

 puzzle, which becomes still more confusing when, as in the Narcomedusa?, some species 

 are parasitic and others are not. Each observation then becomes important, and I shall 

 refer to many papers which contain very small additions to our positive knowledge, the 

 present state of which may be summarized as follows : 



1. Some of the Narcomedusa? develop directly from the egg, without asexual multi- 

 plication. 



2. In other species the ciliated larva becomes a parasite upon the body of a totally 

 different medusa, gaining access to the sub-umbrella of Turritopsis, or to the digestive 

 cavity of a Geryonid. It there multiplies asexually; producing, by budding from an 

 aboral stolon, other larva? which are at first hydras. These hydra larvae become con- 

 verted irt^Lv. ?;,\i'dusa? by direct metamorphosis. 



3. Similar Cunina larva? arc found in the stomachs of many species of Cunina. In 

 some cases the larva? become converted into Cuninas with the specific characteristics of 

 the adult which carries them, but in other cases they differ in the number of tentacles 

 and sense organs, and in other particulars. The youngest of these larva? are free and 

 ciliated, while the older ones are attached and produce buds from an aboral stolon. 



4. No one has shown, by careful examination, that any adult Cunina produces buds 

 from its stomach or from any other part of its body, and there is every reason for be- 

 lieving that the Cunina larva? found in their stomachs are parasites, like those found 

 in Turritopsis and in Geryonids, and that a Cunina larva, found in the stomach of an 

 adult Cunina, does not necessarily belong to the same species with the adult. 



So far as I am aware Krohn was the first to observe a Cunina larva. In a paper 

 which was published in 1861 (11), he says that he found at Messina, in 1813, great 

 numbers of tentaculated larva?, fastened by their aboral surfaces to the protruded gast- 

 rostyle of a Geryonid, Qeryonia f>roboscklalis. He gives few details, and appears to 

 regard the larva? as the asexual progeny of the Geryonia. 



In 1851, Johannes Muller (71) captured at the surface of the ocean at Marseilles 

 great numbers of small ciliated larva?, and a series of older stages which were sufficiently 

 complete to satisfy him that the larva is the young of a very simply organized Narco- 

 medusa, JEginopsis (Solmundella) mediterranea. 



As the youngest larvae arc ciliated, he believed that they are newly-hatched egg- 

 embryos, and as each one of them becomes converted into a medusa, he suggests that 



