554 HYDKOZOA (2.) The medusiform persons being early produced did not separate themselves from the colony, but the whole colony became free (if it ever were fixed), the medusiform persons carrying the hydriform persons away with them. Thus the highly differentiated swimming and floating colonies of the Siphonophora originated. (3.) The medusiform persons ceased to detach themselves from the fixed hydriform persons or colonies, and developed the ova and sperm within themselves, whilst still small in size and attached to the hydriform stock. Having once abandoned the detached, free-swimming life, the medusas underwent in different genera a varying amount of degene ration and atrophy, of which we have in existence all FIG. 18. Two female sporosacs (degenerate medusae) of Ilydractinia echinata. (From Gegenbaur, after Van Beneden.) a, ectoderm ; 6, encloderm ; o, egg- cells; g, enteric cavity. In A an invagination of the ectoderm, which is more complete in B, represents the rudiment of the sub-umbrella space. ber of Hydromedusce (figs. 35, 38, 39, 40, and 42) have lost all evidence of the real characters of their medusa-forms, just as others have suppressed the evidence of their hydra-forms by direct development from the egg ; and inasmuch as both these processes take place in genera having the closest affinity with genera in which both hydra-form and medusa-form are fully preserved, it is not possible to erect groups similar to the Haplomorpha of Carus or the Monopsea of Allman for their reception. The difficulty of classification is, however, rendered very great, for a double system becomes necessary, which shall deal with the characters of hydriform and medusiform persons in parallel equivalent series. The difficulty is considerably enhanced when we find that iden tical medusa-forms may spring from unlike hydra-forms, and, conversely, that closely allied hydra-forms may give rise to very different medusa-forms. The character first noticed by Rapp as distinguishing the hydruid polyps from the coral-polyps, namely, that of developing their genitalia as external bodies (Exoarii) instead of internally (Endoarii), FIG. 17. Diagrams illustrating the gradual degeneration of the medusa bud into the form of a sporosac. The black represents the enteric cavity and its con tinuations; the lighter shading represents the genital products (ova or sperm). A, medusiform person still attached by a stalk at the aboral pole to a colony (phanerocodonic gonophor of Allman) ; B, modified medusiform person, with margin of the disc (umbrella) united above and imperforate (mouthless) manu- brium (adelocodonic goimphor of Allman); C, sporosac, with incomplete extension of the enteric cavity into the umbrella, rudimentary invagination above to form the sub-umbrella cavity; I), sporosac with manubrial portion only of the enteric cavity; E, sporosac without any trace of manubrium. possible degrees, leading from the fixed " phanerocodonic gonophors" (Allman, bell-like genital buds) of many Siphonophora through the " adelocodonic gonophors " (genital buds with the bell no longer open but closed by the union of the margins of the disc) of Cordylopliora to the sporosacs of Hydractinia, and even to the simple genital warts of the little degenerate Hydra viridis of fresh waters (see fig. 17, and explanation). By this process a large num- . B is seen by the considerations just adduced to be fallacious. The Hydromedusce, it is true, often (not always) develop their generative products from the ectoderm, and the geni talia frequently project as ridges and discharge themselves directly to the exterior in this division. The Hydromedusoe contrast in this respect with the Scyphomedusce and An- thozoa, which develop their genitalia from the endoderm, and are (to use Rapp s terms) Endoarii whilst the former are Exoarii. But the bodies mistaken for external generative organs by Rapp and other early observers in many hydroids, and in Hydra itself, are aborted degenerate medusae. (4.) A further set of changes, which have affected the original hydriform colonies and their medusa-buds so as to produce new complications of structure among the Hydro medusoe, are summed up under the head of " polymorphism." The differentiation of hydriform and medusiform persons is a case of dimorphism ; a further distribution of functions, with corresponding modification of form, gives us "polymor phism." Polymorphism is unknown in the Scyphomedusve, and it is chiefly confined to two groups of Ilydromedusce (the HydrocorallincB and the Siphonophora In the hydriform colonies of Ilydractinia (one of the Gymnoblastea-Anthome- diis(v) the outer hydriform persons of the colony (fig. 39) differ in form from the rest, and have wart-like tentacles. In the same genus, and also in many CalyptoUastea, the hydri form persons which are destined especially to give origin to medusa-buds are devoid of tentacles and mouth, and are known as blastostyles (Allman), (fig. 43). In Hydro- corallince (fig. 53) elongated hydriform persons (dacty- lozooids) with no mouth and sporadic tentacles are set in series around a central short mouth-bearing person (gastro- zooids) forming the " cyclo-systems " of Mr Moseley (figs. 52 and 55). In the Siphonophora, in addition to nutritive (hydriform) persons and generative (medusiform) persons, there may be rows of swimming-bells (medusae devoid of mouth and of genitalia), covering-pieces (flattened medusae), and tentacle -bearers (hydrif own persons with one long highly- developed tentacle), (see figs. 56 and 57). Hypothesis of the Individuation of Organs. The building up of complex individualities, such as a hydrozoon colony, a flowering plant, or a segmented worm or arthropod in any one of which a number of common units are repeated, but with varied form and function in each part of the com pound body is generally admitted to be explicable in two ways, and which of the two explanations may be adopted in any one case must depend on the ultimate inference from a wide series of observations. The first hypothesis, which undoubtedly applies to the ordinary hydriform colonies of Hydrozoa, to the segments of Taenia, and to plants formed by the repetition of phyllomes, is that an original unit like those which constitute the composite organism has freely budded, and repeated its own structure in the well-marked units which remain conjoined to form an aborescent or linear aggregate. This is " eumerogenesis," and such aggregates may be termed eumeristic. By a division of labour and consequent modification of form among the units of a eumeristic aggregate, such an aggregate may (in the course of phylogeny) acquire varied shape and definite grouping of its constituent units, and a high speci alization as an individual. The high degree of individua- tion which may be thus attained is due to the more or less complete synthesis of a eumeristic colony. The more highly individuated Chaetopods and Arthropods are syn thesized linear colonies. The cyclo-systems of the Hydro- coralline? are undoubted examples of synthesized colonies. The second hypothesis is one which is applicable to cases which, in the absence of special evidence to the contrary, might be regarded as highly synthesized colonies. Accord ing to this second hypothesis, such highly individuated
composite organisms have not (in their phylogeny) passed
