i8o 



THE EVOLUTION OF SEX. 



In another direction, namely, among the true jelly-fishes (Acraspedd), where 

 an active habit greatly preponderates, we still find the occurrence of asexual 

 multiplication. Some forms (for example, Pelagid) are entirely free ; at the 

 opposite extreme a few (Lucernarida) may be described as sedentary; between 

 these we find the common aurelia, which settles down in its youth, and gives 

 rise by division to what afterwards become the large sexual jelly-fishes (see 

 fig. p. 187.) 



(^ 



Fig. 55. — Siphonophore Colony, showing the float (a), 

 the swimming-bells {b), and the nutritive, repro- 

 ductive, and other "persons" beneath. —From 

 Lang, after Haeckel. 



There remains two classes of crelentrates — the Ctenophora, like Beroe, 

 which represent a climax of activity, and never divide ; and the Actinozoa 

 (sea-anemones and corals), which lead to a passive terminus again, and exhibit 

 profuse asexual multiplication. A few sea-anemones divide normally, just as 

 they may be multiplied by artificial cutting. Fragments may also be given off 

 in an arbritrary sort of fashion, reminding one of the overflow buds of sponges. 

 The division may be either longitudinal or crosswise in sea-anemones, and the 

 budding of corals takes many forms, resulting in the quaint complexity of 

 brain-corals and the like. In one sea-anemone {Gonactinia prolifera), where 

 transverse division occurs, it is interesting to notice that this has only been 

 observed in young forms with undeveloped sexual organs. It recalled, in fact, 

 the asexual multiplication of a young jelly-fish. In another of the corals 

 {Antipatharia) Brooks has recently observed how a nutritive "person" may by 

 constriction form a reproductive individual on either side. 



Worms. — The lower worm-types are roughly distinguishable from most of 

 the higher by the broad fact that they are all of a piece, without rings or seg- 



