18 AMEKICAN HYDiiOIUS. 



Again, how does it come about that two species {Obelia longissima and Ohelaria gelatinosa), 

 so nearly ahke that they have repeatedly been mistaken for each other by experienced writers, 

 differ so in their gonosomes that the former produces free medusfe with 1 6 marginal tentacles 

 and the other produces sessile gonophores from which planulse are produced wliich develop 

 outside of the gonangium?' 



DEVELOPMENT OF THE CAJMPANULARIDyE. 



The origin of the sex cells is much like that described in the first part of this work in con- 

 nection with the Plumulaiidae. Weismann^ has discussed this piocess in Cmnpanulana jlexuosa 

 and in Gonoiliyrxa loveni with his usual thoroughness. He finds that the ova originate in the 

 endoderm of the stem in both of these species, later migrating into the blastostyle and finally 

 into the gonophore. The spermatozoa, however, seem to have a different origin in the two spe- 

 cies, arising, in Campanulana flexuosa, in the ectoderm of the blastostyle and of the ultimate 

 twigs on which the gonangia appear. In Gonofhyrsea loveni the male sex cells arise in the endo- 

 derm of the branches. In this species Weismann found sterile gonophores even where the 

 meconidia had been developed. In some cases male colonies bore gonangia with four to seven 

 gonophores, all entirely destitute of spermaries. 



The origin and maturing of the male sex cells in' Campanulana jlexuosa has been worked 

 oiit in great detail by Thallwitz.^ He confirms Weismann's description as to the origin of the 

 spermatoblasts in the ectoderm 'of the ultimate twigs which bear the gonangia and described 

 their development with great minuteness. 



Hartlaub ■* has made a careful investigation of the origin of the sex cells in Ohelia. He 

 studied more particularly 0. adelungi, a species found in Helgoland, and ascertained that sex 

 cells are found in the ectoderm of the manubrium of the medusa, but that they really originate 

 in the endoderm, passing into the ectoderm when approaching maturity. In the later stages 

 of their development the ova move from the manubrium and take their final position under the 

 radial canal. He finds that the sperm cells originate as indifferent sex cells seen at the first 

 day of the free Ufe of the medusa, and are located at the 1)ase of the manubrium, and, extending 

 for one-thii'd of the length of the radial canals, finally take their place in the spermaries, where 

 they mature as sperm cells in the ectoderm. There thus appears to be fundamental agreement 

 in the origin and growth of the ova and spermatozoa in tliis species of Ohelia. 



The Campanularidse do not differ materially fi'om the other families thus far discussed in 

 the growth of the ovum into the free-swimming planula. 



The formation of the colony after the fixation of the planula has been investigated with 

 great care by Alfred Kiihn,'' who has worked with the following species of Campanularidje : Ohelia 

 geniculata, 0. dichotoma, Gonothyrsea loveni, Campanularia Jlexuosa, and Clyiia joJiTistoni. The 

 following account is condensed from that given by Kiihn : 



The planula after settUng to the bottom becomes transformed into the primary hydranth 

 with a chitinous perisarc covering the pedicel and the hydrotheca covering the hydranth. Below 

 the latter are the usual annulations of the perisarc. Below these annulations the coenosarc is 

 pressed outward against the perisarc, producing a bulging appearance at this point and incUcating 

 the location of the first budding ("spooss spitz") of another hydranth. By a repetition of this 

 process the entire colony is built up." In the development of the hydranth from one of these 

 buds, the bud is prolonged until it forms a tubular process of coenosarc covered mth perisarc 

 and usually annulated proximaUy (fig. 65). Later, the distal end of the tube is enlarged so 

 as to form a cup-like expansion (fig. 66). The top as well as the sides of this cup, or hydrotheca, 



' Die Hydromedusen Helgolands, 1897, p. 489. 

 ' Die Entstehung der Sexiialzellen bei den Hydromeijusen, 1883. 



^ Ueber tUe Entwickhmg der miinnlichen Keimzellen bei den Hydroiden, Jenaische Zeitschrift, vol. 18,1885, p. 390. 

 ■* Beobachtungen uber die Entstehung der Sexnalzellen bei Obelia, 1884. 

 ■' Sprosswachsthnm und Polypenknospung bei den Thecophoren, 1909. 



" Die ganze Kolonie besteht also aus einer Anzahl von aufeinanderfolgendeji Polypen, von deneji jeder durch 

 Knospimg vom vorhergehenden aus entstanden ist. 



