176 



KNOWLEDGE. 



[Septembeb 1, 1891. 



brightness of the Earth, when in " opposition," as seen 

 from Venus, brighter than Venus at its greatest brilliancy 

 as seen from the Earth in the ratio of !J0 to 57. 



Taking the diameters of the Earth and Moon as 7912 

 miles and "2163 miles respectively, the areas of their 

 apparent discs would be in the ratio of 13-38 to 1. Hence, 

 with the same " albedo," the Earth and Moon, as seen 

 from Venus, would differ in brightness by 2-81 stellar 

 magnitudes. 



Now Plummer found that Venus at its greatest 

 brilliancy is nine times brighter than Sirius. The Earth, 

 therefore, as seen from Venus, would appear (-9-^-1 14-21 

 times or 2-8H stellar magnitudes brighter than Sirius. 

 The Earth and Moon would therefore shine as two stars, 

 one about half as bright again as Venus at her brightest, 

 and the other about equal to Sirius, and separated, when 

 the Moon is in " quadi-ature," by about 81 minutes of arc. 

 forming a superb " naked eye double star," perhaps the 

 finest sight in the planetary system. They would present 

 the appearance of a " doiable planet," in striking contrast 

 with the faiutness of the other satellites of the Solar 

 System. The Earth would show a disc of about 62" in 

 diameter, and the Moon one of about 17", and the 

 markings on both might be well seen with a good 

 telescope. 



Seen from Mars, the Moon would also be visible as a 

 small attendant planet to the Earth, but varying con- 

 siderably in briUiancy owing to its phases. 



The Moon's title to rank as a planet rather than a 

 satellite is strengthened by the fact that her path in space 

 is, like the planetary orbits, always concave to the Sun. 

 Professor Young says in his "General Astronomy" that 

 " if we represent the orbit of the Earth by a circle of 

 100 inches radius, the Moon would only move out and 

 in a quarter of an inch, crossing the circumference 25 

 times in going once round it." This is a very different 

 arrangement from the satellites of -Jupiter and Saturn, 

 which seem to form miniatures of the Solar System. 



HYDROID ZOOPHYTES. 



By Herbert Ingall. 



FORMERLY the study of Zoojjhytes could only be 

 carried on at the sea-shore, but now continued 

 observations of many of the lower forms of marine 

 life can be made when far away from the sea by 

 means ot a marine aquarium at home : a better 

 knowledge of the conditions under which sea-water can be 

 kept renders it possible to store up subjects for study for 

 many months, or indeed years, at a distance from the coast. 

 Among the smaller objects of marine hfe the Hydroid 

 Zoophytes will always claim attention, not only from the 

 beauty of their form but from the interesting and extra- 

 ordinary changes they undergo during their life-history. 



Who has not observed the delicate feathery masses 

 thrown up on the beach after a storm or heavy ground 

 swell — masses often of considerable size, but of an ex- 

 ceedingly fine horny substance — or noticed, when ex- 

 amining some clear weed-fringed pool among the rocks on 

 a bright day, dehcately curved white plumes hke feathers ; 

 or seen among the tufts of weed a fine, brown, wiry-looldng 

 branching stem, the tips of the branches terminating with 

 a sort of pink flower, which latter if watched will at 

 mtervals be seen to bend and nod ? These are some of the 

 Hydroid Zoophytes, which formerly were supposed to be a 

 sort of link between animals and vegetables, but they now 

 are without hesitation placed in the animal kingdom. 



Three facts will be noticed in the general plan of growth 

 of the Hydroids. There is first the general and common 

 flesh of the composite animal called the ctrnomn-, which 

 may be in the form of a creeping thread, naked, or covered 

 with a cliitiniiux or horny substance, which latter is almost 

 always present in the arborescent or brandling forms and 

 serves to support them ; or the canomrr may be present 

 merely in the form of a thin filament connecting the 

 separate polyps, but in any case it answers the same 

 purpose, which is the general connection of the whole mass 

 of polyps forming the individual colony, and the circulation 

 and conveyance of nutriment over the whole. Secondly, 

 the iiolyiiUcK or flower-like animals, that develop at the 

 ends of the branches in the arborescent forms, or may bud 

 out at any part in those that have the canosan- adherent to 

 stones or shells : these serve to collect and catch food, and 

 even living prey, by means of thread-like arms or tentacles. 

 And, thirdly, the reproductive or sexual buds, called the 

 (joiKiphoreti, in which are developed the elements for the 

 propagation of a new colony : the object so produced is 

 called the yonmooid. 



I have had under observation for many months a colony 

 of Podocoryne earned : they were obtained at Hastings last 

 September. In passing through the old town I noticed 

 some fresh specimens of Dog Whelk {Sassa ntitulata) 

 lying in the roadway, and, seeing that they had evidently 

 not long come from the sea (no doubt having been just 

 thrown out of some fishing boat), I placed them in my 

 collecting jar, from which they were transferred the same 

 day to an aquarium at home. 



A curious association exists between many of the 

 lower animals quite dissimilar in structure — the connec- 

 tion of Podocorrae with the Dog Whelk (Xassa) is almost 

 constant, and the obvious explanation would seem to be that 

 the rough shell and the roving life of this whelk is favour- 

 able for the development of the polyps ; the scavenging and 

 carnivorous habits of the Dog Whelk must be the means 

 of a much more constant supply of food to the lower 

 animal than it could obtain alone. The presence of an 

 allied form f Hi/dnntinia j on shells inhabited by the Hermit 

 Crab, may also be ascribed to similar causes, but it is 

 somewhat singular that they should each keep to their own 

 host ; we seldom find Hydractinia on Nassa, or Podocoryne 

 on Pagurus, at least so far as the writer's experience 

 extends. 



The Polyps composing the colony form an elegant cover- 

 ing to the somewhat sombre-coloured shell of the mollusc 

 over which they wave about like a delicate feathery veil, 

 the beauty of which is enhanced by the lively habits of the 

 Dog Whelk. 



The Poh/pitex are of a deUcate white colour (presenting 

 the appearance shown in Fig. 1). They are furnished with 

 an opening at the upper end, and eight 

 delicate arms, or tentacles, situated a 

 little below and around the mouth ; these 

 serve to arrest any small particle of food 

 or minute animals, and to bring them to 

 the mouth, but it is very probable that 

 many or all of these lower animals are 

 not altogether dependent on the solid 

 food caught by the tentacles ; extremely 

 minute organisms are always present in 

 the sea, and they are most likely ab- 

 sorbed by the walls of the digestive cavity. The body of 

 Podocoryne is very extensUe (as are also the tentacles), 

 and it is attached at its lower end to the catwsarc, a 

 delicate creeping covering of fleshy matter which, in this 

 case, clo.sely follows the indentations and reticulations 

 of the shell, quite avoiding the elevated parts or bosses, 



