ASEXUAL REPRODUCTION. I 93 



respect is the freshwater sponges {Spongilla) ; they have also been described 

 in other common sponges, e.g. , in Clione, the borer in oyster shells. 



Coelenterates. — In such names as zoophytes, sea-firs, sea-roses, there 

 is a prevision of the undoubtedly plant-like character of many of the 

 coelenterates. A sessile habit is very general, though often only a phase 

 in the life-history, and 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 approximate equality of internal and external 

 conditions. One becomes relatively overfed, another starved. Slight 

 differences of function gradually become emphasised and exaggerated, till 

 division of labour is established. The structural aspect of this is differen- 

 tiation or polymorphism among the members of the colony, and results in 

 the establishment of nutritive and reproductive, sensitive and protective, 

 " persons." Thus in the common Hydractinia, the open-mouthed nutritive 



One of the acarids or lice {Glycipha^us cursor) forming a life-saving cyst, while the 

 individual itself dies. 



individuals are markedly contrasted with the dependent reproductive 

 persons ; and again, in different form, the rhythm repeats itself in the contrast 

 between active, offensive, and sensitive elongated members, and entirely 

 passive and abortive spines, which form a chevaux-de-frise under shelter of 

 which 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 pre- 

 dominant 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 ( Trachy- 

 medtisce) 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 raffaelii)., 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 towards the Portuguese man-of-war, or 



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