774 THE ANIMAL KINGDOM. 



permanent hypoblast is differentiated from this central mass of cells as a superficial 

 layer of cells beneath the epiblast, the central cells undergoing gradual absorption. In 

 the Calycophorid Galeolariafiliformis=Epibulia aurantiaca, a nectocalyx, a polypite, 

 tentacle, hydrophyllium are formed. The second nectocalyx appears on the pedicle 

 of the first group of zooids, i.e. on the developing coenosarc. The buds of a second 

 polypite, tentacle, hydrophyllium, grow out, and the colony is established by the 

 development of successive groups of zooids in the order named. The cavity of the 

 first nectocalyx in Hippopodius is remarkably small, but its mesoglaea extends as a 

 cap over one extremity of the planula. A provisional hydrophyllium is developed in 

 the first instance in most Physophoridae, Athorybia, Physophora, Agalmopsis, and 

 Crystallodes, but not in Halistemma. The float appears next in succession; see 

 note, p. 772, ante. Agalmopsis and Crystallodes have a set of provisional hydro- 

 phyllia (the so-called Athorybia-stego), and the tentacle knobs of the young Physo- 

 phorid are different in character to those of the adult. Halistemma has a single 

 provisional nectocalyx. A portion of the planula persists for some time as the yolk- 

 sac of Crystallodes. 



A well-developed diffuse system of ganglion cells has been detected in the 

 Discoideae. Ganglion cells occur also in the tubular coenosarc of some other 

 Siphonophora. The nervous elements are especially differentiated in Physophora. 

 Remarkable branched cells, ' neuro-muscular cells,' in connection with the longi- 

 tudinal musculature of the coenosarc, have been described in some forms by 

 Korotneff; cf. Mitth. Zool. Stat. Naples, v. p. 235, PI. 14, Fig. 13. 



The Diphyozooid (supra) swims by means of the medusiform sexual zooid. 

 If it is derived from a hermaphrodite colony, it is said nevertheless to be itself uni- 

 sexual. But one derived from the unisexual Abyla pentagona has been observed 

 by Chun to develope first a male and then a female zooid. 



The sexual products of the Discoidean medusa are developed after the medusa 

 is detached, therefore probably from the ectoderm of the manubrium ; in all other 

 Siphonophora properly examined from the endoderm : in Epibulia (= Galeolaria) in 

 the rudiment of the gonophore : in Hippopodius in the bud of the future peduncle of 

 the bunch of gonophores : in Forskalia in the rudiment of the peduncle : and in 

 Agalma in the rudiments subsequently differentiated into several gonophores. The 

 young sexual cells are not sub-epithelial at first, as they are in those Hydroidea 

 where they appear in the endoderm, but they project into the cavity of the bud : 

 but whether or no they really originate in the first instance from endoderm cells is 

 not known. The sexual cells in the male wander into the ectoderm of the manu- 

 brium : so too in the female of the Calycophores named, but the endoderm sub- 

 sequently grows round each ovum, furnishing it with an incomplete follicle placed 

 to the outer aspect. But in the Physophores Forskalia and Agalma the single 

 ovum is arrested in the endoderm, and surrounded by the spadix as stated p. 771. 



Chun has propounded a theory of cyclical development in Monophyidae ; e. g. 

 (i) a form called by him Monophyes primordialis, which has been shown by Claus to 

 be a larva, gives origin to (2) a second {orm=Muggiaea Kochii, in which the swim- 

 ming bell of the Monophyes is replaced by a second and differently shaped necto- 

 calyx, and the coenosarc developes groups of zooids : and (3) one of these groups 

 of zooids is detached as a Diphyozooid, the sexual Eudoxia Eschscholtzii. Cf. 

 Chun, SB. Akad. Berlin (2), 1882, and A. N. H. (5), xi; Claus, Arb. Zool. Inst. 



