Notes on some Hydromedusae from the Bay of Naples. 573 



of well knowu cases, such as Hi/dractinki, Podocori/ìie, etc. But iu 

 PerigoniìHus^ a geuus comprising perhaps the largest number of 

 species amoug (Tvmuoblasteae, so far as detailed Information is 

 available it seems that alniost witbout exceptiou the species are 

 essentially commensal. 



In common also with most known species the one here under 

 consideration has a gelatinous perisarc, which covers the entire col- 

 ohv from the hydrorhiza to the body of the hydranths, and covering 

 the latter to the bases of the tentacles, though not in any appreci- 

 able degree extendiug to the tentacles, as seems to have been the 

 case with Weismaxn's species. There can hardly be reasonable 

 doubt that the mucus-like covering is a definite secretion by the 

 hydroid itself, and homologous with the chitinous perisarc commou 

 to hydroids generally. 



A character in the preseut species is the long, slender, stem- 

 like branches which arise at sharp angles from the stems and grow 

 quite long without signs of giving rise to hydranths. There seems 

 to be evidence that eventually hydranths arise from their tips, but 

 it is not conclusive, as in only a few cases have newly formed 

 hydranths appeared upon them. 



The following are the more distinctive diagnostic characters of 

 the species, which seems to be new, and for which the name P. 

 napolitanus is proposed: — 



Trophosome (PI. 22 Fig. 25): Hydrocaulus composed of numerous 

 stems, variously brauched, some of the latter without hydranths, ali 

 branches arising at sharp angles from the main stems or branches, 

 entire hydrocaulus arising from a net-like hydrorhiza, and attaining 

 a height of from 4— S mm. Hydranths elongate, more or less fusi- 

 form, with conical hypostome, and with a single whorl of rather 

 stout tentacles. 7 — 12 in number. Entire colony provided with an 

 indefinite, gelatinous perisarc, extending over bodies of hydranths 

 to dose beneath the cirele of tentacles. Perisarc sometimes dense 

 and somewhat chitiuous iu older portion of stems and hydrorhiza, 

 aud of dull browuish color. (ìelatinous perisarc often studded with 

 foreign particles, diatoms, etc., which thus give to the colony a 

 rough, or fuzzy appearance, as shown in fig. 25. 



(lonosome: wholly unknown. 



It is worthy of mention in this connection also, that I took on 

 May 4. 1903, a hydroid which, barriug the absence of gonophores, 

 bore a dose relation to P. repens Wright, and is probably identical 



Mittheilungen a. d. Zoùl. Station zu Neapel. Bd. 16. 3S 



