14 ■ AMEKICAN HYDEOIDS. 



whirh preponts a bilateral symmetry more pronounced than any other that I have seen among 

 the campamilarian liydroids. The species was first described by Hincks/ and his excellent 

 description has not been improved upon by subsequent writers. It is here quoted: 



They [the gonaiigia] are perfectly hyaline and of a unique and singularly graceful form. They are best described 

 us slipper-shaped; but the iipper extremity is ciu'ved into the most exquisite spiral, while the lower portion tapers 

 rapidly toward the point of junction with the ringed stem. Immediately below the spiral a wide opening leads into 

 the tubular passage by which the embryos make their escape, which bends upward within the capsule and terminates 

 in a circular orifice near the top. The gonophore.'?, which are numerous, form an elongated mass nearly filling the cavity 

 of the gonotheca. 



The curious introverted neck of these gonangia is a feature that is unique, so far as the 

 writer laiows, among the hydroida. 



The gonophores. — These are entirely homologous with those commonly found among the 

 Sertularidse. The blastostjde usually occupies a central or axial position in the gonangium, 

 and the gonophores arise as hernia-like protrusions from the blastostyle. As in the Sertularidse, 

 the end of the blastostyle is commonlj^ expanded into a "plug" which fills the distal end of 

 the gonangium. In Oampanularia the ova or sporosacs are borne by gonophores which are 

 budded directly from the blastostyle, and the ova develop directly into planulse which later 

 escape from the gonangium by rupturing the hd of the latter, or the Ud is ruptured by some 

 other means. In other genera in which there are no free medusae the same general morphology 

 of the gonophores prevails, the difference being largely in the number of gonophores which are 

 produced as buds from the blastostyle. In Oampanularia (fig. 61) these are numerous and 

 usually borne on all sides of the blastostyle, while in Gonothyrsea they are few and are borne 

 on one side of the blastostyle (fig. 58). In all cases the distal gonophores are the older, and there- 

 fore discharge their sexual products in regular succession from above downward ; or, rather, the 

 growth of the blastostyle itself carries the gonophores from below upward as they mature. 



In the genus Ohelaria we find an exceptional state of affairs, in that the gonophores produce 

 sexual products as usual, but the ova develop into planulse outside of the gonangium, instead 

 of inside as in Oampanularia, the most nearly related group. ^ The single species of Ohelaria, 0. 

 gelatinosa, was, until Hartlaub's work appeared, regarded as an Ohelia, and its medusae were 

 described by numerous writers as having 16 tentacles at the time of liberation. This probably 

 arose from the fact that this species was earty confounded with Ohelia longissima. 



In the genus Ohelia the gonophores are produced on all sides of the axial blastostyle and 

 occur in great numbers, being very small in size, and often completely cover the blastostyle 

 (fig. 4S). The medusae are discoid in shape, with four radial canals and eight hthocysts borne 

 on the inner sides of the tentacle bases. The amazing number of medusae giA^en off from a single 

 colony when there are hundreds of gonangia each hberating scores of these minute medusae 

 may account in part for the great abundance and extensive geographical distribution of many 

 species of this genus. 



In Clytia (fig. 49) the gonophores are not so numerous, as a rule, as in Ohelia, but they are 

 arranged aroimd a central or axial blastostyle, as in that genus. The medusae are usually larger 

 than in Ohelia and ai"e at first almost globular m shape, with four radial canals and eight htho- 

 cysts placed between the tentacle bases. After liberation their form is more nearly hemi- 

 spherical. 



In Orthopyxis the blastostyle is branched and bears but one mature medusa at a time, and 

 this one is relatively very large and more or less globular or campanidate or hemispherical in 

 shape. The medusae are so large as to crowd the blastostyle to one side, and are remarkable 

 from the fact that they have neither manubrium nor tentacles, but agree with the medusae of 

 Ohelia and Clytia in having eight lithocysts. 



Tlie medusx. — Many campanularians differ from the PlumularidEe and Sertularidae in the 

 fact that they produce free-swiniming medusas wliich are A^ery generally regarded as naotUe 



' Zooph. of South Devon and Cornwall, Ann. Mag. Nat. Hist., ser. 4, vol. 8, 1871, p. 78. 



^ Hartlaub has di.scussed this point at length in his Hydromedu.sa3 Helgolands, 1897, p. 489. 



