HYDROZOA CRASPEDOTA. 



773 



The nectocalyces of the Physophoridae, when present, come next to the 

 float. The rest of the coenosarc carries the other zooids which are 

 arranged in distinct groups in the Physophorid Apolemia, and in all Caly- 

 cophoridae. Segments bearing several groups of zooids are detached in 

 Apolemia, and single groups (bract, polypite, tentacle) are set free in 

 some species of Diphyes, in Abyla, Monophyes, and Muggiaea. These 

 single groups are known as Diphyozooids, and have received special names, 

 e.g. Diplophysa (Monophyes\ Eudoxia, &c. (Diphyes}. The Diphyozooid 

 developes a succession of sexual zooids. The nectocalyces and zooids are 

 developed along one and the same aspect of the coenosarcal tube ; but, 

 owing to the fact that it is spirally twisted in Physophoridae, they appear to 

 be disposed in two or more rows, and the real arrangement is not evident. 

 The twist of the nectocalycine region is in the opposite direction to that 

 of the rest of the coenosarc. New nectocalyces in the Physophoridae 

 appear distally to the float : the other zooids distally to the nectocalyces 

 both in Calycophoridae and Physophoridae^. The coenosarcal tube is 

 eminently contractile. Its ectoderm developes a circular and a longitu- 

 dinal layer of muscle cells, its endoderm a circular. 



The colonies of Physophoridae are mostly of moderate size, but 

 Agalma elegans attains a length of four feet, Apolemia uvaria of twenty 

 to thirty feet. The Calycophoridae are small, the largest, e. g. Diphyes, is a 

 few inches long, Galeolaria two feet or less. The colony is retractile 

 within a furrow or canal formed at the side of the distal nectocalyx in 

 Diphyes and A by la ; by the two nectocalyces in Pray a ; or into a pit in the 

 side of the single nectocalyx in Monophyidae. The Siphonophora are found 

 in the Mediterranean and the open seas, sometimes at considerable depths 

 (eight hundred to fifteen hundred fathoms or even more). Physalia is 

 brilliantly coloured : Velella and Porpita are greenish blue. The majority 

 are hyaline with spots of colour on the float, the polypites, hydrocysts, and 

 tentacles. 



The ovum appears to be impregnated and to develope while floating freely. It 

 is devoid of a membrane save in Hippopodius. Segmentation is regular, and results 

 in the formation of a solid ciliated planula with large vacuolated central cells. The 



1 It appears that in the Diphyidae the functional nectocalyces are constantly replaced by others. 

 The new nectocalyx is produced distally to the old, not proximally as in Physophoridae, See 

 Chun, SB. Akad. Berlin, 1885, pp. 521 et seqq. ; and Korotneff, Mitth. Zool. Stat. Naples, v. 1884, 

 pp. 279 et seqq. 



The Hippopodidae differ from other Siphonophora in the fact that the nectocalycine section of 

 the coenosarc is bent parallel to the section bearing the remaining zooids, and the point at which the 

 flexure takes place is also the one at which the rudiments of both nectocalyces and other zooids are 

 formed. Other notable peculiarities in the Order are the presence of hydrocysts among the necto- 

 calyces of Apolemia, and the addition of a nectocalyx to each group of zooids in certain species 

 of Praya, hence erected by Chun into a genus, Lilyopsis (SB. Akad. Berlin (i), 1885). 



