ZOOLOGICAL SOCIETY BULLETIN. 



975 



irregular outgrowths on the surface. The sea- 

 biscuit (Suberites) and similar species which 

 grow on sandy or niuddv bottom have long- 

 horny fibres which penetrate the sea bottom to 

 hold the sponges in position. In the Venus 

 flower-basket and glass-sponge the fibers are 

 siliceous, and in tiie latter these take the form 

 of a twisted rope of very stiff bristle-like fibers 

 which may be a foot or more in length. These 

 are modified spicules of the sponge, just as in 

 the sea-biscuit they are modified spongin fibres. 



Coelenterates. — The common fresh-water hy- 

 dra is ordinarily held in place by an adhesive 

 base, but if food becomes scarce or other con- 

 ditions improper the animal liberates itself and 

 crawls off in search of more favorable sur- 

 roundings. The sea-anemone fastens itself in 

 the same manner and is capable of a very slow 

 creeping movement. The majority of hydroids 

 attach themselves by means of a horny secre- 

 tion and the same is true of the sea-fans or 

 gorgonias. Such forms are never able to free 

 themselves. The corals and the millepores are 

 made fast by means of a 

 calcareous secretion a n d 

 these also remain perma- 

 nently fixed. Nearly all of 

 the jellyfishes are free- 

 swimming at all times, but 

 the primitive Leucernaria 

 has the ability to attacli 

 itself at will by means of 

 an adhesive disc. The lar- 

 val stage of Aurelia and 

 certain other jellyfishes is 

 sessile as a polyp for a pe- 

 riod, in which condition it 

 is known as a "scyphula." 

 Scvphulae, probably those 

 of the common jellyfish 

 Aurelia flavidula, have lived 

 for some years in the bal- 

 anced salt-water aquaria at 

 the New York Aquarium. 

 They propagate freely by 

 sexual budding, but have 

 never metamorphosed into 

 the adult sexual condition. 



Rotifers. — The tiny wheel- 

 animalcules are either free 

 living, fixed or parasitic. 

 There are two distinct meth- 



ods of fixation among those which attach 

 themselves. In one case there is a pincer- 

 like organ at the posterior end of the body 

 and with this apparatus the rotifer may 

 anchor itself to any object, but such forms rarely 

 retain the grasp for an}- great period. The 

 other method is by means of a cement gland 

 opening upon a basal expansion and species 

 which have this method of attachment usually 

 remain fixed as long as the conditions of life 

 are satisfactory. Some of these build very 

 beautiful cases into which they can retract the 

 body when in danger. 



JVorms. — Numerous parasitic worms are either 

 temporarily or permanently fastened to the body 

 of the host by hooks or suckers, or by both at 

 once. The adult tapeworm as an intestinal par- 

 asite, is attached to the mucus membrane, and 

 when once it obtains a hold, never loses it. In 

 some eases the rostrum is provided witli re- 

 trorsely curved hooks which catch in the mem- 

 brane, or the cells of the lining wall of the 

 intestines are drawn into the suckers on the 



SEA-ANEMONES, WHITE CORALS AM) HYDROIDS 

 One^half natural size. 



