302 FISHES 



hydroid zoophytes, and these probably serve to hide it even 

 better than appendages which only resemble natural growths 

 of the sea-bottom. The hydroids are of a species (Stylactis 

 minoi) which has been found only on this fish, and the fish has 

 never been found without the hydroid. The zoophytes are not 

 parasitic, they feed by means of their own mouths and tentacles 

 and absorb no nourishment from the fish. The case is therefore 

 one of commensalism, the two organisms probably benefiting 

 one another reciprocally, the hydroid by being carried about 

 from place to place and so getting more food, the fish by being 

 concealed or disguised ; it is possible that the fish is also directly 

 protected, the hydroids being distasteful and inedible. With 

 regard to the origin of this invariable association, it is evident 

 that the hydroid could not grow on the fish unless the latter 

 were very sedentary in its habits, but it is difficult to under- 

 stand why only one species should grow on it, and not grow 

 elsewhere. A Darwinian would probably hold that a particular 

 variation occurred which could only survive under these peculiar 

 conditions ; a Lamarckian, on the other hand, might suggest that 

 in the course of generations the peculiar mode of life caused 

 changes in the hydroid which made it a distinct species. 



Association between young fish and large jelly fishes or 

 medusae is of common occurrence in European seas. One of 

 the earliest naturalists to notice this was the Norwegian, G. O. 

 Sars, who, when he was investigating the life-history of the 

 cod in 1867, found the young of that species when an inch and 

 a half in length were to be found almost exclusively under the 

 umbrellas of the medusae, which were very abundant. This 

 observation was made off the Lofoten Islands in the north of 

 Norway. Sars found that the young cod were feeding on the 

 crustacean parasites of the jelly-fish. The young of the scad, 

 Cavanx trachunts, exhibit the same habit. The young of Tet- 

 ragonurus, a Mediterranean fish belonging to the Percesoces, 

 have been observed to take up their abode in the respiratory 

 cavity of large Salpae, animals which have the transparency 

 and the mode of life of jelly-fishes, but which are related to the 

 sea-squirts or Ascidians, and therefore to the Vertebrates. 



Another very remarkable case of association between fishes 

 and other animals is that of Fierasfer and its allies which live 

 in the bodies of sea-cucumbers or Holothurians (Fig. 24). 



