POLYPI. 137 



fleshy fossae, but the whole of the soft part of the polypary is con- 

 tracted and withdrawn into the harder basal portion.* ]Mr. Darwin 

 relates of the Virgularia — a long and slender kind of sea-pen, which 

 he saw in vast numbers projecting like stubble above the surface of 

 the muddy sand — that when touched or pulled they suddenly drew 

 themselves in with force, so as nearly or quite to disappear. The 

 slender calcareous axis supporting the fleshy beds of the polypes of 

 this sea-pen becomes softer and bent towards its base, and the elasticity 

 of this part contributes, with the expansion of the irritable tissue, to 

 enable the Virgularia to rise again through the mud.f 



Ovaria and sometimes tortuous filamentary secreting organs (y) 

 analogous to the testes in the Actinia, are developed from the meso- 

 gastric folds in the chylaqueous cavity. In the anthozoic polypes 

 of the red coral most of the individuals are androgynous, and present 

 both the ova and the spermatozoa ; but a few instances occur in 

 which some polypes develope only the one kind of generative pro- 

 duct, and others the other kind. In those singular forms of com- 

 pound polypes, called Pennatulce, or sea-pens, the ova are found in 

 the pinnae or branches of the common stem or axis of the polypes. 

 But groups of individuals have been found in which only the sperm- 

 sacs are developed.^ In the Veretillum a delicate network of vessels 

 conveys the nutrient fluid prepared by each polype to the common 

 connecting parenchyme of the entire compound animal. 



In the Gorgonia verrucosa Cavolini states § that the ova, which 

 are developed in an ovarium situated at the base of each polype, 

 doubtless in the mesogastric folds, are discharged by eight small pores 

 ■which open between the bases of the eight tentacles. The ova escape 

 as ciliated "planulae" of an ovoid form, which emerge from the 

 ovarian pore with their small end foremost, then turn and swim with 

 the great end foremost. In the month of June a Gorgonia, six inches 

 in height, discharged ninety such larvag in the space of an hour. 

 They first rose spirally towards the surface of the water, then swam 

 horizontally. They have the same property as the ciliated planute 

 of the marine Hydrozoa of changing their form by contraction of 

 their tissue, a property which the ciliated zoospores of sponges and 

 algae do not possess. When the larvae of the Gorgoniae rested they 

 attached themselves to the vessel containing them by their larger 

 end. The ciliated embryos of the Caryophyllia calycularis are also 

 discharged by small apertures at the basal interspaces of the tentacles 

 of each polype. H They are of a deeper red colour than those of the 

 Gorgonia, have the same locomotive power, and show the same 



* CXXIV. p. 2. t CXXK. p. 99. % CXV. p. 101. 



§ CVI. pp. 47. 50. tab. iv. figs. 7—10. and 13—15. || CVI. p. 56. 



