ASEXUAL REPRODUCTION. 179 



form gemmules, which often save the life of the otherwise dying sponge. They 

 are complex enough, with sheaths and spicules, and sometimes even with a 

 float, but' in principle they simply do by a multiple union what is otherwise 

 attained by ovum and sperm. Best known in this respect is the freshwater 

 sponges (Spotigilla); they have also been described in other common sponges, 

 for example, in Clinoe, the borer in oyster-shells. 



Ccelenterates.--ln such names as zoophytes, sea-firs, sea-roses, there is a 

 prevision of the undoubtedly plant-like character of many of the ccelenterates. 

 A sessile habit is very general, though often only a phase in the life-history, 

 and an asexual reproduction runs "riot. A well fed hydra is prolific in bud- 

 bearing ; and numerous gradations connect this with the myriad colonies 

 exhibited by many hydroids. The individuals forming a united family share 

 in the common life and nutriment. As the colony becomes complex, it is often 

 physically impossible for all the members to remain on terms of even approxi- 

 mate equality of internal and external conditions. One becomes relatively 

 overfed, another starved. Slight differences of function gradually become 

 emphasized and exaggerated, till division of labor is established. The struc- 

 tural aspect of this is differentiation of polymorphism among the members of 

 the colony, and results in the establish- 

 ment of nutritive and reproductive, sen- 

 sitive and protective, "persons." Thus 

 in the common Hydractinia, the open- 

 mouthed nutritive individuals are mark- 

 edly contrasted with the dependent repro- 

 ductive persons ; and again, in different 

 form, the rythm repeats itself in the con- 

 trast between active, offensive, and sensi- 

 tive elongated members, and entirely 



, , . . . . , , Fig. 54. — One of the acarids or lice (Glyci- 



passive and abortive spines, which form phagns curspr) forming a , ife . saving cyst( 



a chevaux-de-frise under shelter of which while the individual itself dies, 



the others cower. It is usually supposed 



that the sessile hydroids are in a sense degenerate from more active ancestral 

 types. The free-swimming embryo becomes exhausted, settles down, and 

 exhibits predominant vegetativeness with postponed sexuality. In many cases, 

 however, there is a recovery of the ancestral liberty of action, for modified 

 "persons" are set adrift as active, free-swimming, sexual medusoids. 



There are, however, active forms of the true medusoid type ( Trachymcduste) 

 which never descend to the sessile nadir of existence, but yet exhibit the asexual 

 tendency of the class in forming temporary clusters of pendent buds. Lang has 

 lately described a remarkable compound medusoid {Gastroblasta raffae/ii), 

 which has sometimes as many as nine stomachs, and may be assumed to be 

 highly nutritive. The remarkable point, however, is that the compound adult 

 is the result not only of continued budding, but of a process of rectangular 

 incomplete division. Along with some others it leads on toward the Portuguese 

 man-of-war, or siphonophore series. Here the larva develops at first into a 

 simple medusa-like individual, but this buds off a manifold series of " persons," 

 which, by dislocation or even migration, becomes arranged in all the beauty of 

 the siphonophone colonies, which surpass even Hydractinia in their division 

 of labor. It is difficult enough in some cases to distinguish between true 

 "persons" — which Haeckel calls "Medusomes"- — and mere organs like 

 protective bracts, which are also budded off. 



