AMERICAN HYDROIDS. 



Jin in !lu and of Diplitma rosaccn; Thallwitz 1 describes the origin of the sex cells 

 in *S', /iiil/ii; l/ii j>nli/::nii!iis; de Vareune 2 published a careful account of the reproduction of 

 Sertuluriti jut/, //'/</; Allman, in the introduction to his Report on the Challenger Ilydroids, 

 discussed the gonosome of Si/iitlni-inin (p. xxv), and the origin of the sex cells in >'< r1nl<iri<i 

 />u in Hi/ (p. xxxm); Levinsen carried on important investigations regarding the operculum, 

 and published the results in 1892 and 1893; 3 Hartlaub, in his masterly work. 4 discusses 

 some morphological problems, such as the operculum, hydrothecal teeth, and gonosome of 

 Serf nl</ r< lln, although these discussions are merely incidental to the main purpose of the work, 

 which is almost purely systematic. 



One of my students, Mr. J. H. Paarmann, has done some excellent work, which has not been 

 published, on the operculum. To these works I shall have frequent occasion to refer. 



TROPHOSOME. 



The general form of the colony in Sertularid;e is pinnate, the branches usually being on 

 opposite sides of the main stem, the branches themselves being opposite or alternate. Simple, 

 uoncolonial forms are entirely lacking in this group, there being no such a thing as a mature 

 hydranth with its hydrotheca entirely independent of others, 5 as is found frequently in the 

 campanularians among the Calypteroblastea, and a number of tubularians among the Gymno- 

 blastea. As a result we iincl the usual outcome of the communistic idea, the subserviency of the 

 individual to the colony as a whole, and a tendency toward a bilateral rather than a radial 

 arrangement of branches. This may account for the fact that nowhere among the Sertularida- 

 do we find a true verticillate arrangement of the branches, such as is found among the Campanu- 

 laridie \\\ C<.tinpn)ndari(t /'< /ficillitt<t, and among the Plumularida? in Antennularia <inl< /////////. 

 There are, however, a number of cases where the branches spring from all sides of the stem, as 

 in several species of Tlnnarin. In these cases we have the pinnate arrangement of branches 

 modified by torsion into a spiral which may be rendered more or less obscure by the irregularity 

 of the branching, but which is quite evident in Tim/aria tlmj<i, for instance. A more open 

 spiral is found in a number of species, illustrated by the very graceful colony of Sertnlfn'ni 

 ii/-i/i ni, it and HydraXLmania f alcana. Unbranched colonies are uncommon in this family, although 

 several are found in Xv///A//vW, especially in the Desmoscyphus group, as, for instance, S. xt<>i>l-,i/i 

 (Plate V, fig. 6), and in the genus Sertularella several of the rugosa group, as, for instance, 

 S. rni/iiKii. are often unbranched. 



Among the Sertularida' are found the only species of hydroids that have regularly anasto- 

 mosing brunches, which sometimes form a rude mesh or network, as in DicfyoclacKum^abellum,' 

 and here also occur the only examples of hydrothecal branch origins that 1 have seen in the 

 order. This is illustrated in the genus Thecocladium, not represented in American waters, and 

 also by sporadic eases which are not infrequent in the genus Sertula/rella^ as, for example, S. 

 dentifera Torrey (Plate XXV, fig. 1) and AH, //////,/ graciUs Nutting (Plate XXXV, fig. 1). 

 In all probability, however, this occurs occasionally in other groups, such as the Campanularidse, 

 although the present writer has not seen it there. 



In size no sertularian yet reported reaches the maximum found among the Plumularida'. 

 (See Part I, p. -i.) The largest colonies that I have seen were those of Tlm'mr'm cupressina, 

 that attain a height of about 18 inches under favorable conditions. Kirchenpauer, however, 

 reports specimens from the mouth of the Elbe that are 2 feet in height/' This is perhaps the 

 maximum size reported for any sertularian. As a rule the}' are very much smaller, from 1 

 to 4 inches being the ordinary proportions. Many, of course, are much smaller than the 



Menaische Zeitschrift, XVIII, 1885, p. 426. 



2 Reeherches surla reproduction des Polypes Hydraires, 1882, p. 27. 



3 Viilenskabelige Meddelelser fra den Xaturhistoriske Forening i Kjobenhavn, 1892, p. 22, and 1893, p. 41 et sei|. 

 Mil-vision der Sertularella-Arten, 1900, pp. 10-12. 



5 An apparent exception to this statement i.s found in the case oi Sertularella solitaria, a new species described 

 beyond, which bears single hydranths cm pedicels. It is possible, however, that the single known specimen may be 

 immature and that the adult nilnny may resemble that of K.fu 



"See Hincks, British Hydroid Zoophytes, 1868, p. 272. 



