NERVOUS SYSTEM AND BEHAVIOUR 457 



of a species at certain stages of its life-history. Such activities, to the 

 observer, appear directed towards the survival of the individual or the 

 reproduction of the species. They are frequently complex in nature, and 

 appear to be evoked by specific stimuli to which the individuals of a species 

 are inherently susceptible. 



Like the majority of benthic animals, polychaete worms are usually 

 restricted to particular kinds of substratum, characteristic of each species. 

 In some instances the proper substratum, inhabited by the species, is 

 selected by the pelagic larva. An example is provided by the mitraria larva 

 of Owenia fusiformis, which has a pelagic phase of some four weeks, dur- 

 ing which time it swims with an upward tendency. When ready to meta- 

 morphose it swims towards the bottom and moves along slowly in contact 

 with the latter. If it encounters a bottom of fine sand or grit, similar to 

 that in which the adults live, the larva responds by settling down, meta- 

 morphosing and building a tube. Larvae of Notomastus and Ophelia like- 

 wise select a substratum corresponding to that in which the adults occur; 

 an important factor in inducing settlement of Ophelia larvae is the presence 

 of suitable micro-organisms in the substratum (126). 



These polychaete larvae show inherent susceptibility to particular 

 environmental stimuli, which call forth a specific pattern of response. 

 Similarly, we find adult polychaetes which show preferential selection of 

 environmental habitat on the basis of some particular stimulus. Certain 

 commensal polynoids, for example, react positively to specific chemical 

 influences from a particular host species, ignoring other species closely 

 related to the host (Chapter 14). Many polychaetes are tubicolous and 

 build tubes of characteristic form and composition. When material from 

 the substratum is incorporated in the tube, such material is selected in 

 accordance with specific preference, and the tube is moulded by the worm 

 into a characteristic shape (p. 656). 



Selection of food, although not as extensively studied in marine as in 

 terrestrial species, likewise shows preferences suitable to the animal's habits 

 and feeding apparatus. Some species, such as octopuses, hunt moving prey 

 and depend on patterns of visual stimuli. Others are tactile feeders, e.g. the 

 sole (Solea soled) which feels the substratum with cheek villi. Others, 

 again, react to chemical clues, e.g. the whelk Buccinum, a carrion feeder 

 which finds its food by smell. The feeding behaviour of these animals is 

 innately determined, and to find its full expression it must be triggered by 

 particular stimuli. The same specificity of response is shown sometimes in 

 escape- or avoiding-reactions to natural enemies. Certain gastropods and 

 scallops, for example, have a common pattern of escape behaviour to 

 carnivorous starfish, such behaviour being quite dissimilar from that 

 elicited by other non-predatory animals. The response is evoked by a 

 chemical stimulus emanating from the starfish, or by contact with the tube 

 feet ; the mollusc then takes to flight, either by leaping, crawling or swim- 

 ming, according to the habits of the species. More specific and complex in 

 their manifestations are the reproductive habits of many species, e.g. of 



M.A.— 15* 



