974 



ZOOLOGICAL SOCIETY BULLETIN. 



tribution, after which they become permanently 

 fixed and are altogether different in appearance. 

 The free-swimming stages of many animals 

 were described as distinct species in many cases 

 until the life history became known. 



In some groups there is a definite alternation 

 of generations ; one stage fixed and reproducing 

 sexually, followed by a free generation having 

 sexual organs. Thus in the hydroids there 

 are polyps which reproduce by budding. Some 

 of the buds break loose from the colony and 

 undergo further development into the sexual 

 stage known as the medusa or jellyfish. 



In many cases a temporary attachment is 

 necessary for the development of the individual. 

 Thus while most crinoids remain fastened after 

 attachment, in the genus Ante don the individual 

 severs the connection after a time and again 

 becomes free. The same thing is true of the 

 genus Fungia among the corals. In the mol- 

 lusca, the common scallop, after passing through 

 the free-swimming larval stage, attaches itself 

 by means of horny fibres, but after a time it 

 loses this connection and thenceforth remains 

 free. In the fresh-water clam, also, a larval 

 free-swimming stage (known as the Glochidi- 

 um) is present, but the larva then attaches it- 

 self to the gills or skin of a fish and leads a 

 semi-parasitic life for a time, following which 

 it again breaks away to lead an independent 

 existence for the rest of its life. Among the 

 amphibia, the young tadpole of the frog, after 

 hatching from the egg, fastens itself for a 

 short time to the stem of a water plant or 

 similar object, by means of suckers around its 

 mouth. After a short period this attachment 

 is severed and the suckers are shed. 



Among the forms which may attach or re- 

 lease themselves at will are certain protozoa, 

 such as Stentor; some coelenterates, such as 

 Hydra and Leucernaria; the species of rotifers 

 (wheel-animalcules) which possess a grasping 

 foot; many species of worms provided with 

 suckers ; numerous Crustacea which attach them- 

 selves by their appendages; and fishes, such as 

 the lamprey eels, which fasten themselves by 

 a sucking mouth to the bodies of other fishes 

 or to stones; the lump-suckers (Cyclopteridae), 

 sea-snails (Liparidae) and the remoras (Eche- 

 neidae) which are provided with sucking discs 

 formed from modified fins. 



Protozoa. — The great majority of the one- 



FINGER SPONGE (chalina'arbuscula) 

 This specimen, a foot in height, was attached to a pebble. 



celled animals are free-moving, and this is true 

 even of the parasitic species, most of which are 

 able to move about to some extent within the 

 body of the host. The best known examples 

 are Stentor, already alluded to as anchored at 

 will, and Vorticella, which is ordinarily fixed 

 but which may, in the presence of untoward 

 conditions, break away from the stalk by which 

 it is attached and swim into a new locality, 

 where it may re-attach itself and secrete a new 

 stalk. In the colonial Epistylis, the attachment 

 is permanent. A remarkable growth of this 

 form was recently figured covering a crayfish 

 from Prospect Park Lake, Brooklyn. (See the 

 Bulletin of the New York Zoological Society, 

 No. 54, Nov., 1912, p. 927.) 



Sponges. — These animals generally maintain 

 their position by means of adhesive secretions 

 of the cells in contact with the substratum. The 

 sulphur sponge burrows into the substance of 

 shells disintegrating them and forming nodular 



