LOCOMOTIVE POLYZOA. 



455 



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 beautiful crowii 

 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 ex- 

 tremity of a tentacle still more highly mag- 

 nified, at E, in order to show the cilia by 

 which the necessary 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 present are known to exist. As may 

 be seen by reference to the engraving, which 

 represents a specimen slightly magnified, the 

 entire polyzoary is not only free and un- 

 attached to any object, but even possesses 

 the power of locomotion. 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 polyzoary 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 

 .surprising 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 



POLYZOA, viu. 



A. Alcyonidium gdatindsuit*. 



B. Alcyonidium gelatinosum. x (Tentacles of a single 

 polype.) C. Pedicellina echinata. x 



D. Pedicellina echinata. (Natural size.) 

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

 The sign x signifies that the object is magnified. 



