334 EVENINGS AT THE MICROSCOPE. 



Polypes, which at length divide themselves into colonies 

 of Medusae. * 



At first you will perhaps see nothing remarkable in 

 another object which I collected in my rock- ramble to-day. 

 A Hermit-crab in an old Natica shell ; both common 

 things enough. Yet look more narrowly. The greater 

 portion of the shell is not smooth, has no such porcelain- 

 like polish as the Natica usually has, but is clothed with a 

 sort of downy nap, a coarse sponginess of a greyish hue, 

 splashed with yellowish and pink tints. The shell is in- 

 vested with Hydractinia. 



We restore the strange partnership, shell, fleece, and 

 crab, to the glass of sea-water ; where we soon see the 

 whole tumbling about the bottom in uncouth agility. 

 Assist your eye with this pocket-lens, and look again. 

 The shaggy nap upon the shell now bristles with tall 

 slender polypes, crowded and erect, like ears of corn in a 

 field. 



No high magnifying power is necessary to furnish us 

 with considerable entertainment from this populous colony. 

 The polypes stand individually nearly half-an-inch in 

 height ; each consists of a straight slender column, sur- 

 mounted by eight straight rod-like tentacles, four of which 

 stand erect, slightly diverging, and the other four alter- 

 nating with these at their origin, extend horizontally like 

 the arms of a turnstile. 



The rough jolting of the crab over the stones the ex- 

 panded polypes bear with equanimity ; they are used to 



* Since this work was published, many efforts have been made to 

 explain, and to bring into harmony with what were supposed to be the 

 laws of generation, these strange facts, which seem to subvert all our 

 notions of individuality. Steenstrup's phrase is now by common 

 consent abandoned : there is no alternation of generation proper. 

 That hypothesis which regards the phenomena as those of metamor- 

 phosis, the fixed condition (Laomedea) being analogous to the larva, 

 the free-swimming (Medusa) to the adult, is one of the most plausible ; 

 but an attempt to apply it in all the cases shows it to be untenable. 

 (See Greene, Ccelenterata, p. 76). 



