30 



AMERICAN HYDROIDS. 



..eet 



end 



kt' 



94 



aborted, and the mouth seems closed, while the ova are developing. In this case the gonophores 

 spring directly from the body walls of the hydranth, the latter being thus a true blastostyle. 



Reasoning from analogy, we are justified in regarding the blastostyle of the plumnlarians as 

 degenerate persons, although this view can not be demonstrated by direct observation. There 



are reasons, however, for believing that a gonophure itself, 

 in this group, as in many others, is really a degenerate 

 medusa, or " plauoblast," as Alliuaii would call it. 



Weismanu found in his investigations of the origin of 

 the sex cells of Plumi/lnria hitlccioidcs and Aittcinndnrio 

 that the arrangement of the cell layers in the gonophores 

 of these species was such as to strongly suggest that of 

 the cell layers of the planoblast. ' In the former species, 

 which was the first studied, he found that there were four 

 layers of ectoderm, instead of one, outside of the ova. 

 These four layers he regards as an equivalent of the three 

 ectoderm layers and one endoderm layer which would be 

 found in making a cross section through one side of the 

 bell and inaunbrinm of a medusa, within a gonangium, the 

 section ending at the place where the ova are situated in 

 the planoblast; that is, between the stut/Jamelle and the 

 ectoderm of the mauubrium. A comparison of Weismann's 

 figure (fig. 9o) with a diagram of a medusa within a gonan- 

 gium showing the plane of the hypothetical section will 

 make Weismanu's ideas plain (fig. 94). It will be noticed 

 that the order in which the several layers occur in his figure, 

 counting from without inward, is ekt, ekt', cntl, ekt", ekt'", 

 which stand respectively for the external ectodcrmal layer 

 of the gonophore, the external ectodermal layer of the bell 

 of the medusa, the endoderin of the bell, the subumbrellar 

 ectoderm layer of the medusa and the ectoderm of the manu- 

 brium. There is also a space between eld" and ekt'" which 

 Weismann regards as the space between the bell and the 

 manubrium. The parallelism is therefore exact and goes 

 far to prove the correctness of his idea regarding the honiol- 

 ogy existing between the gonophore and the planoblast. 



Weismauu does not snow in his figure that the layer 

 entl differs histologically from the others. In the text, 

 however, he says : " There can be no more doubt that the 

 present pluinularians descend directly or remotely from the 

 medusa bearing hydroids, and it is not impossible that, at 

 present, plumularians exist with medusa broods." 



A single blastostyle may bear a single gonophore, as 

 in the genus AglaopJieitin, or several, as in many species of 

 Plnmulnria. In Plintuilaria ecltiiiulata two or more gono- 

 phores are borne in succession upon the same part of the 

 blastostyle. All of the cases that I have been able to find 

 in which there are more than one gonophore on a single 

 blastostyle have been eleutheroplean forms, and I have 

 seen no statopleau with more than one gonophore to a 

 blastostyle. 



Ordinarily the colonies are unisexual, but there are 

 a few cases such as Antennularia and Plumularia catharina in which both sexual products will 

 be found in the same colony. When mature, the sexes can easily be determined, if the gonophores 

 are removed from the gonaugia, by the fact that in the male the spadix is surrounded by a 



entl 



HISTOLOGY OK THE GONOPHORES OB' PLUMULARIDjE. 



Fig. 03. Vertical section of tbe gouangium of Pin- 

 tiiularia pinnata. c, cavity ofgonophore; c', cavity 

 of blastostyle ect, ectoderm; end, endoderm ; err, 

 floating granules in cavity of blasioatyle; n, 

 " deckenplatte " of Weismanu; p, perisarc; sp, 

 sperm cells; st, " stntzlamelle." 



Fig. 94. Diagram in longitudinal section of hydroid 

 medusa in a gonangium, to shmv arrangement of 

 bist.ological layers. Let terin^ as in rijj. 95. 



Fig. !t5. Partial transverse flection of the jrmiopboroof 

 riiimiilaria lial'doides; eiz, ovum; cW, external 

 ectodermal layer of gonopbore; ekt', emlodcrm of 

 bell; fkt", Bubnmbrellar ectoderm layer of Hie me- 

 dusa; ekt'", external ectodermal layer of bell of 

 null uaa; ent, endoderm; entl, ectoderm of manu- 

 brium; st, stiit/.lamellc. 



1 Die Eutsteliimg der Sexualzellen bei den Hydroweduseu, Jena, 18S3, p. 185, pi. xxiv, fig. 12. 



