ZOOLOGICAL SOCIETY BULLETIN. 



973 



ATTACHED ANIMALS. 



FREE movement in animals seems to possess 

 so many advantages over the fixed or ses- 

 sile habit of life that one unacquainted 

 with the facts would hardly suppose that so 

 large a number of aquatic animals would find 

 it advantageous to attach themselves during 

 some portion of their life. Yet some groups 

 like the sponges, bryozoa and brachiopods are 

 characteristically fixed, and numerous protozoa, 

 the majority of coelenterates, a few worms, 

 some echinoderms, many mollusca. some Crus- 

 tacea and many ascidians show this habit. A 

 few fishes are able to attach themselves at will 

 and some larval amphibians are fixed for a 

 short period. This leaves only a iew minor 

 groups of invertebrates and the higher verte- 

 brates in which no examples of the attached 

 habit appear. 



The attachment may be permanent after fix- 

 ation, or it may be merely temporary, and in 

 some cases it may be assumed at will. The 

 advantage of such fixation is not always clear, 

 and it is certain that it is not the same in all 

 cases. In many marine animals living near the 

 shore-line, attachment, either permanent or tem- 

 porary, may serve to prevent the animal from 

 being washed ashore in rough w r eather. In 

 many forms, such as the encrusting corals, 

 bryozoa and ascidians, it undoubtedly aids in 

 protection from the predaceous enemies. In 

 parasitic species the reason 

 is clear enough. In certain 

 cases permanent fixation 

 seems to have been arrived 

 at through crawling or 

 creeping stages ; in others it 

 has come about through 

 temporary attachment. 



The means of fixation are 

 as variable as the groups in 

 which this habit occurs. It 

 may be by means of an ad- 

 hesive secretion, by a suck- 

 ing disc, by hooks or spines 

 as in some parasitic spe- 

 cies, by branching root-like 

 structures, by horny or cal- 

 careous matters, by cellulose 

 as in the ascidians, or by 

 special grasping organs as 



may be noted in certain rotifers and crustaceans. 



Several different methods of holding fast ma}- 

 occur within the same group. Thus the coelen- 

 terates may have an adhesive base, as in the 

 common hydra and sea-anemone; there may be 

 a horny secretion as in the case of most hy- 

 droids, and this may take the form of an ex- 

 panded base, a trailing adherent rhizome, or of 

 radical fibres, while in the corals and millepores 

 the secretion is lime. In the mollusca attach- 

 ment may be secured by means of a lime secre- 

 tion as in the common oyster; by horny fibres 

 as in the salt- water mussel ; by the expanded 

 foot as in many gastropods, or by suckers as 

 in the temporary attachment of the octopus. In 

 the Crustacea it may be by a calcareous or horny 

 secretion as in the acorn and goose barnacles 

 respectively ; by a branched, root-like absorbing 

 organ as in the parasitic barnacle Sacculina, or 

 the copepod Lernaea; by a tube as in the para- 

 sitic barnacle Duplorbis, or by hooked thoracic 

 appendages, as in the parasitic isopods and 

 some copepods. 



It is evident that for purposes of distribu- 

 tion a free-moving stage must sometime appear 

 in the life history of every form. Such a time 

 occurs always in the larval life and, as a rule, 

 a striking metamorphosis follows the attach- 

 ment of the free stage. Thus the larvae of 

 sponges, corals, oysters, barnacles and bryozoa 

 swim about for a time and accomplish their dis- 



CRIMSON SEA-ANEMONE (.Tealia erassicornis) 

 Note the basal disc by which it is attached to the stone. 



