THE PLUMULARIDvE. 7 



Tlie greater part of the tubes, liowever, in a number of species could be traced directly to the 

 hydrorhiza, and formed in the aggregate almost, if not quite, the entire mass of root filaments. 

 The apparent contradiction between previous writers and my own investigations may be recon- 

 ciled if it can be demonstrated that the hydrorhizal elements and the hydrocladia are themselves 

 homologically equivalent; that they are interchangeable terms in the life history of individual 

 colonies. 



Bale, as above stated, gives ample proof that the accessory tubes are true hydrorhizal elements. 

 Professor Verrill, in a letter to the writer, says: "It has long been recognized that basal stolons 

 are homologous with branches and hydrocladia." It would be difficult to point out any fundamental 

 difference between stolons and hydrorhiz.e, and indeed it seems to me proper to regard the former 

 as merely modified elements of the latter. 



Another fact bearing on this point was noted by me while studying at the Marine Biological 

 Laboratory in Plymouth, England, 1 where I saw Hie entire process by which the fully matured 

 hydrocladia of Flinnularia pinntita were converted into stolons from which new colonies arose. 

 We thus see that the hydrorhizal elements called stolons are converted into accessory tubes (for 

 example, many species of Cladocarpus); that hydrocladia are converted into accessory tubes (C. 

 paradisea) ; that accessory tubes are converted into branches and give rise to hydrocladia (many 

 species, according to Bale); and, finally, that hydrocladia are converted into stolons from which 

 new colonies arise (P. pinnata). 



As a matter of fact, the subject of homology among Hydroida has been unnecessarily obscure 

 because the whole group is so primitive that any one part is homologous with several others, or 

 rather that the parts are not greatly or fundamentally differentiated. For instance, the creeping 

 rootstock may properly be regarded as a portion of the hydrorhiza in many species; in many 

 others it is considered a true stem, or hydrocaulus, which has adopted a procumbent habit; 

 again, as wo have just seen, it takea the guise of an accessory tube in a fascicled stem, which may 

 in its distal portion adopt the further disguise of a branch or even a hydrocladium. 



Each tube of the fascicled stem is made up of the ordinary elements of a simple stem. There 

 is the outer layer of perisarc, within which is the cienosarc, composed, as usual, of ectoderm and 

 endoderm with the. intervening stutzlamelle, and finally the central axial cavity lined with flagel- 

 late endoderm cells. In addition to the lateral communications between the accessory tubes, 

 Spencer has found in Plumiilnria prociimbens rimilar connections between the hydrocladiate and 

 adjacent accessory tubes. 



In many fascicled stems there are certain portions where the whole stem looks as if it had been 

 violently compressed or pinched. This is notably the case in the genus TluTocarpun (fig. 5). I 

 am unable to explain these curious structures, which iliucks 2 unhesitatingly declares to be arrested 

 branches. ' ' On the back of it (the stem) at nearly equal distances are formed little regular arch- 

 like risings which are compressed and hollowed in the middle' (Ellis). This is a very accurate 

 description of these curious prominences, which have been supposed to mark the stages of growth. 

 They are formed by an occasional divergence of a portion of the tubes from the ascending line of 

 the stem, and are, in fact, arrested branches.'' 



A careful dissection of a much-branched specimen of Thecocarpus (Aylaoplienia) niyriophyUitm 

 throws considerable doubt upon this. The facts ascertained are as follows: The hydrocladiate 

 tubes of the stem give origin to the hydrocladiate tubes of the branches, part of the accessory 

 tubes of the former accompanying the latter. This being true, the protuberances or pinched 

 places should contain a branch from the hydrocladiate tube of the stem if the protuberances are 

 suppressed branches. A close examination fails to disclose any such state of affairs. On the con- 

 trary, the protuberances are on the opposite side of the stem from the hydrocladiate tube, which 

 is not at all diverted from its course, nor does it give origin to any branch at that point. 



Of course it is possible that these protuberances indicate growth periods in the life of the 

 colony. The regularity of their appearance would seem to indicate that they are normal slruc- 



1 Sec Notes on the Reproduction of Pltimulariaii Hydroids, American Naturalist, Novemlu-r, IX! LI; anil .Juiirnal f 

 the Marine Biological Association of the United Kingdom, IV, 1-06. p. 152, where a detailed account of my obs.-rva- 

 tions on stoloniferons reproduction will le found. 



- British Hydruid Zoophytes, London, ISDN, p. 291. 



