ZOOLOGICAL SOCIETY BULLETIN 



977 



forming what is known as the byssus, such as 

 in the common salt-water mussel. The fibres 

 are secreted by a gland in that portion of the 

 body known as the foot in the free-moving 

 mussels and clams. In the second case, typified 

 by the common oyster, the under valve is at- 

 tached directly by a secretion of lime, often so 

 firmly that it is difficult to dislodge it. The 

 oyster is said to trap the raccoon sometimes by 

 catching his foot between the valves of its shell. 

 In the third case, which is common to many 

 of the univalves or gastropod mussels, there is 

 an expanded disc or foot which, in attachment. 

 acts like a sucker. In most cases these forms 

 move about by means of the same organ. Thus 

 the common water-snails hold fast by this foot 

 and move slowly along in search of food. The 

 limpet is a very much flattened gastropod which 

 as a rule moves about but little. It is astonish- 

 ing how firmly these animals can retain their 

 hold. The ear-shell (Haliotus) of the Pa- 

 cific coast holds to the rock so firmly that 

 a large one has been known to trap an animal 

 attempting to feed upon it. In one recorded 

 case a coyote inserted his snout under the shell. 

 when the haliotus closed down upon him hold- 

 ing him a prisoner. The octopus which uses 

 its suckers for holding its prey also uses these 

 same structures for holding onto rocks. So 

 powerful are these suckers that the weight of 

 the animal may be lifted by the suction of a 

 single disc. 



Crinoids: The sea-lilies are now found only 

 at considerable depths in the ocean and are 

 with a few exceptions permanently fixed to the 

 bottom by means of a jointed calcareous stalk 

 which is cemented to a rock or other object by 

 an expanded base composed of a calcareous se- 

 cretion. 



Crustacea: Numerous methods of fixation 

 have been evolved in this group. The parasitic 

 copepods and isopods usually retain their hold 

 upon the body of the host by means of hooked 

 appendages, but in some of the copepods. for ex 

 ample, Lernea, there is a branching, root-like 

 sucking organ for attachment. The barnacles 

 ordinarily are fastened by means of a chiti- 

 nous stalk, or by a secretion of lime, but in the 

 parasitic species there may be a branched or 

 tubular absorbing organ. The hermit crab 

 keeps its hold upon the shell in which it lives 

 by means of the last pair of abdominal legs, 



(tOOSE BARNACLES 



Attached to floating timber. 



Much reduced. 



which are especially modified for the purpose. 

 It may release the hold at any time. 



Ascidians: The sea-squirts secrete cellulose, 

 a substance otherwise found only in plants, and 

 by means of this secretion many species attach 

 themselves to the sea-bottom. They grow singly 

 or in colonies, sometimes forming soft crusts on 

 rocks, shells and sea-weeds. The simple as- 

 cidians are sometimes very much appressed to 

 the bottom, but they may be greatly elongated 

 or stalked. In the latter case the base of the 

 stalk is often greatly expanded or divided into 

 finger-like processes for better anchorage. 



Fishes: None of the fishes attach themselves 

 permanently, but in a number of widely sepa- 



ACORN BARNACLES 

 Attached to bark of a pile. 



Natural size. 



