4 LUCERNAIU.E AND THEIR ALLIES. 



components of the organization which it hohls sway over. Thus it is that two, or 

 more, scattered, consimihir parts, or complete organizations may combine to form 

 a seeming one, an apparent, bipartite or muhipartite iinii. The multiple repetition 

 of heads among the lower polymeric kinds is here reduced to a. dual r(?petition, 

 and the parts condensed into one form an approximative unit, a zoological indi- 

 vidiumi, as the highest expression of unity attainable by the vertebrate zoon. 



11. The duality, nay the plurality of the subdivision of the vertebrate axis, as 

 illustrated by the embryo fishes of Lereboullet, is recalled in the diffusiveness of 

 the many hydra; of the dendritic Campanularia;, and is disguised under the inter- 

 minable heteromorpliism of the Siphonophorte ; it is polymerous but dimorphous 

 in Salpa, or polymerous but monomorphous in the fresh-water Polyzoa ; tempo- 

 rarily a polymerous, monomorphic individnum in the fissigemmating Hydra, it 

 eventually resolves itself into disconnected psexido-individua ; for a time polymer- 

 ous, but dimorphic, in the annelidan Myrianida of Milne-Edwards, it finally assumes 

 the appearance of a true, self-contained mdimduum in each one of the separate, 

 independently moving sexual segments, and in the original budding-stock (the 

 direct legitimate offspring of the egg) from which they shot forth. 



§ 2. The hydroid and medusoid CephaJisms. 



12. Under the term cepliallsm we include two forms, or morphs, viz., (1) the 

 cepTialid, or such subdivisions of a body as have a complete organization, whether 

 united in common (as in Spongidse,' some Vorticellida?, Corals, Bryozoa, some 

 AscididiTc, and Pyrosomidse), or separating siu'^^ly from the main stock (as in 

 Hydra and Actiniit) ; and (2) the cephMlold, or those divisions of a fissigemmating 

 body which do not contain a complete organization, and may be either mostly sexiial 

 (as the so-called medusae of Hydromedusae, or the posterior divisions of Myrianida 

 and other worms, or the joints of Taenia, or the Cercaria-brood of Distoma, or 

 the chain of Salpa), or mostly vegetative and sexless (as the hydra? of Hydromedusa?, 

 the Myrianida stock, the head of Taenia, the single, budding stock of Salpa, or the 

 budding Cercaria-nurses of Distoma), 



13. The thorough historian of the multifarious, so-called alternate generations 

 of the AcalepiiiE will see nothing but a generative organ in the spermatic and 

 ovarian sacs of Hydra ; and detect nothing more in the grape-like clusters about 

 the base of the head of Clava, or in the grouped moniliform projections behind 

 the corona of tentacles of Eudendrium. The polymerism of these organs of 

 Eudendrium is nothing more than a repetition of the simple sac of Clava ; the 

 diversity in form is only apparent. But one step higher in complexity and 

 the observer will find in the tentaculiferous terminations of the reproductive 

 sacs of Thamnocnidia and Parypha a premonition of a forthcoming cephalic 

 independence, such as is already fully exemplified in the many hydras of the 

 polymeric, dendritic mass. A similar progression toward cephalic freedom will 



' See the author's memoir on " Spongiffi CiliatiB as Infusoria Flagellata;" Memoirs Boston Soc. 

 Nat. Ili.'it., Vol. I, ISfiT. 



