68 COELENTERATA [CH. 



were enabled to remain suspended, then it would be placed in a very 

 favourable position for getting food, and it has been suggested that 

 the simply floating Siphonophora have thus been evolved from Hydro- 

 medusae. If this view be taken, the three chief divisions of 

 Siphonophora represent three successive stages in the adaptation of 

 the group to a pelagic life. Thus the Physaliidae simply float, 

 the Physophoridae float and swim by nectocalyces, whilst the 

 Calycophoridae have lost the float and trust entirely to their 

 powerful nectocalyces. 



The Siphonophora are remarkable for the varieties of person 

 which compose their colonies. As varieties of the hydroid person 

 may be named the palpons or tactile persons devoid of a mouth, 

 but showing their equal rank with the nutritive person by the 

 possession of similar tentacles. To the category of medusoid persons 

 belong not only the nectocalyces but the bracts transparent 

 sheath-like structures sometimes present, which shelter groups of 

 persons. This extreme variety of persons is foreshadowed in the 

 ordinary Hydromedusae. Hydractinia for instance, which grows at 

 the mouth of whelk shells inhabited by hermit crabs, has palpons 

 amongst its hydroid persons, but in no case is such extreme diversity 

 attained as among the Siphonophora. 



The Hydrocorallinae are really distinguished by the fact that the 

 peris arc which only covers the basal stolons is thick and calcareous. 

 After a while the stolons enclosed in the skeleton die, but fresh 

 stolons are thrown out at higher levels, so the skeleton grows in 

 thickness. The hydroid persons are of two kinds, nutritive persons, 

 gastrozooids, short and with wide mouths, and tactile persons, 

 dactylozooids, which surround each gastrozooid in a circle and 

 which are long and mouthless. Both kinds have short rudimentary 

 tentacles looking like knobs. Most genera produce only gonophores 

 but Millepora give rise to free Medusae devoid of mouth or tentacles 

 in which the genital organs are developed from the manabrum. 



The Sea-Anemones are representatives of a second division of 

 Actinozoa ^ e Coelenterata, which show a decidedly more com- 

 plicated structure than the animals just considered. 

 Unfortunately it is very difficult to obtain the ordinary sea- 

 anemones in a sufficiently expanded condition to make out their 

 structure, since when irritated they contract so much as to 

 throw their internal structures into great confusion. Another 

 animal belonging to the same group is the "colonial" species 

 Alcyonium digitatum, sometimes called "Dead men's fingers." It 



