64 



HYDROZOA 



(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 hydrif orm 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 

 vmderwent in different genera a varying amount of degene- 

 ration and atrophy, of which we have in existence all 



FiQ. 17.— Diagrams Dlnatrating the gradual degeneration of the medusa bud 

 into the form of a sporosac. The black represents the enteric cavity and its con- 

 tinuations J the lighter shading represents the genital products (ova or speim). 

 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 gonophor of Allman); C, sporosac, with incomplete 

 extension of the enteric cavity into the umbrella,— rudimentary invagination 

 above to foim the sub-umbrella cavity; D, 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 Cordylophora 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- 



FiG. 18.— Two female sporosacs (degenerate medusse) of BydracHnia echinaia. 

 (From Gegenbaur, after Van Beneden.) a, ectoderm; 6, endodenn; o, egg- 

 cells; g, enteric cavity. In A an invagination of the ectoderm, which is 

 more complete in B, represents the rudiment of tlie sub-umbrella space. 



ber of ffydromedusce (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 aflSnity 

 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 difiBculty 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 

 difiiculty 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 Eapp as distinguishing the hydroid polyps from 

 the coral-polyps, namely, that of developing their genitalia 

 as external bodies (Exoarii) instead of internally (^recZoam), 



is seen by the considerations just adduced to be fallacious. 

 The Hydromedusm, 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 Hydromedusm 

 contrast in this respect with the Scyphomedusa and .4m- 

 thozoa, which develop their genitalia from the endoderm, 

 and are (to use Eapp's terms) Endoarii whilst the former 

 are Exoarii. But the bodies mistaken for external generative 

 organs by Eapp and other early observers in many hydroids, 

 and in Hydra itself, are aborted degenerate medusae. 



(4.) A farther set of changes, which have affected the 

 original hydriform colonies and their medusarbuds so as to 

 produce new complications of structure among the Hydro- 

 medusoB, 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 Scyphomedusm, 

 and it is chiefly confined to two groups of Hydromedusm (the 

 HydrocorallinoB and the Siphonophora). In the hydriform 

 colonies of Hydractinia (one of the GymnoblasteorAnthome- 

 dusce) 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 Galyptohlastea, 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- 

 corallincB (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 (medusse devoid of 

 mouth and of genitalia), covering-pieces (flattened medusae), 

 and tentacle -bearers (hydrif ormpersons 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 Chsetopods and Arthropods are syn- 

 thesized linear colonies. The cyclo-systems of the Hydro- 

 corallinoB 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 Iiave not (in their phylogeny) passed 



