LOCOMOTIVE POLYZOA. 



455 



waves. In its natural state it is affixed by its base to stones, shells, and other supports, 

 and is always extremely irregular and variable in its form, no two specimens being alike. 

 When picked up, its aspect is anything but attractive, but when placed in sea-water and 

 suffered to remain at rest for a while, it becomes a most beautiful object. From each of 

 the tiny pits with which its surface is thickly studded, projects a polype, with a beauti- 

 ful crown of waving tentacles, and so numerous 

 are these polypes, that they densely cover the 

 surface and render microscopic observation 

 rather difficult. 



As in other species, fresh colonies of the 

 Alcyonidium are formed by gemmules, which 

 are given forth from the general mass, swim 

 about freely for a time, by means of the cilia 

 with which their surface is thickly studded, 

 and when they have attained a proper age, 

 settle down and at once begin to develop 

 fresh cells on all sides. The little vesicles 

 wherein the gemmules are originally formed, 

 may be seen in the spring scattered through 

 the transparent substance of the polyzoary, 

 and looking like little white points. Each 

 vesicle contains about five or six gemmules, 

 and as it can be easily isolated, its rupture 

 and the consequent escape of the gemmules 

 can be easily seen in a moderately powerful 

 microscope. A head of a single polype with 

 its crown of tentacles is shown at fig. B. 



Referring to fig. D on the same plate, the 

 reader will see a group of little objects on 

 footstalks, looking wonderfully like the com- 

 mon moss that grows so plentifully on walls. 

 This is the Pedicellina echinata, of which a 

 magnified group Is seen at C, and the extremity 

 of a tentacle still more highly magnified, at E, 

 in order to show the cilia by which the neces- 

 sary currents are formed in the water for the 

 purpose of obtaining food. 



PASSING to Plate IX. fig. A, we come upon 



one of the most remarkable polyzoa that at POLYZOA vm. 



present are known to exist. As may be seen *.A*y**iaium teiatM***. 



by reference to the engraving, which represents B. Alcyonidium geiatMsum. x (Tentacles of a single 

 a specimen slightly magnified, the entire poly- polype.) c. Pedtctiuua echinata.*. 



zoary is not only free and unattached to any D - PteWina echinata. (Natural size), 



object, but even possesses the power of loco- 

 motion. In the present instance it is shown 

 in an attitude which it frequently assumes, namely, crawling over the stem of some 

 aquatic plant. In order to qualify it for this process, the lower surface of the poly- 

 zoary is modified into a flattened disc, which thus becomes analogous to the foot of the 

 gasteropodous molluscs already described. The substance of the disc is contractile. 



To an ordinary eye, that any creature should crawl, would not appear a very surpris- 

 ing fact, but to the mind of a naturalist, the whole phenomenon is full of wonder. It is 

 easy enough for a single being to advance in a given direction, and even though it has 

 a very army of legs, like a centipede or a julus, the limbs are all directed by the same 

 mind. But in the present case, there is no common centre to which the wills of the 

 myriad polypes that compose the group can be referred ; and the locomotive capacities 

 of the Cristatella remain one of the many unsolved mysteries with which nature abounds. 



D. Pedictllina echinata. 

 E. Pedicellina echinata.. X X (Part of tentacle). 

 The sign X signifies that the object is magnified. 



