ICjO CHAETOGNATHA 



semble many other creatures that dwell at the surface of the 

 ocean in being almost completely transparent. Most species have 

 been taken far out at sea, but some are perhaps rather more 

 numerous near the coast, and one species, Spadella cephaloptera, 

 is littoral. They swim by means of muscular movements of the 

 whole body ; the fins have no movement of their own, and seem 

 to serve as balancers, and not as locomotory organs. Although 

 usually found at the surface of the water, many species have been 

 taken at considerable depths. Chun x states that they are found 

 in countless numbers at depths of from 100 metres to 1300 

 metres. The commonest species at these depths are Sagitta 

 hexaptera and Sagitta serratodentata. Sagitta bipunctata, accord- 

 ing to the same authority, confines itself to the surface. Whether 

 the change of depth is diurnal, or whether it has any relation to 

 sexual maturity, or to any other cause, has not been satisfactorily 

 determined. 



The food of the Chaetognatha consists of floating diatoms, 

 Infusoria, small larvae, and such Copepods as Calamus finmar- 

 chicus, and small Amphipods as Phoxus plumosus? At times they 

 also devour small larval or post-larval fishes, and owing to their 

 incredible numbers, they doubtless do considerable damage to sea 

 fisheries. It is also recorded that they eat one another, and 

 specimens, have been taken which have ingested the whole body 

 of another Sagitta, except the head, which hangs out of the 

 mouth of the eater, and gives it the appearance of a double- 

 headed monster. 3 It has been said that they attack hydroid 

 polypes, but here at any rate they do not have it all their own 

 way. Masterman 4 has figured the apical group of five polypes 

 of Obelia, three of which are engaged in ingesting as many young 

 Sagitta. 



They exist in incredible numbers ; Grassi describes the surface 

 of the sea at Messina on certain days as being literally covered 

 with them, and they must form the food supply of numerous 

 animals which prey upon the pelagic fauna. The immense 

 number of individuals is probably accounted for to some extent 

 by the fact that they lay eggs all the year round, and pass 



1 Bill. Zool. vol. i. 1888-89, p. 1. 



2 Scott, Annals of Scottish Natural History, 1892 and 1893. 



3 E. Beraneck, Rev. Zool. Suisse, vol. iii. 1895, p. 137. 



4 Ann. Mag. Nat. Hist. 6th ser. vol. xiii. 1894, p. 440. 



