66 LIFE IN THE SEA [PART I 



life may be a continual occupation and delight. But in addition 

 no department of biology presents more abstruse problems to 

 those who care for such investigation. 



All the great animal groups with the exception of the higher 

 vertebrates are represented in the plankton. It will be convenient 

 to consider here, in a general sense of course, what organisms are 

 present, premising that there is a very considerable variation both 

 with the locality and with the time of year when the plankton is 

 observed. In the floating microscopic life of the sea there may be 

 distinguished two categories of organisms, (1) the transitory 

 plankton, that is the assemblage of forms which appear only for a 

 short time and then assume other habitats : such are the larvae 

 and young forms of many of the animals and plants living among 

 the benthos and nekton, obviously the place and time of appearance 

 of these will vary with the character of the bottom fauna and that 

 of the nekton. Then we have (2) the permanent plankton which 

 consists of those organisms which live for the whole term of their 

 lives in the sea as drifting creatures, and these too vary from place 

 to place just as the other forms of marine life do. 



. — Fishes appear in the plankton in the form of eggs and larvae. 

 Though, properly speaking, they are nek tic animals, they never- 

 theless make a very obvious part of the plankton in their young 

 stages and some (the skates, rays and dogfishes) belong, while they 

 are developing, to the benthos. All the British skates and rays, some 

 of the dogfishes, and some other fishes, of which the herring is the 

 most important, lay eggs which, being heavier than sea water, sink 

 to the sea bottom, and after remaining there for a certain period 

 of time hatch out. The herring eggs undergo a period of incuba- 

 tion of about a fortnight, at the end of which time the young fish 

 swims out into the water and takes its place among the plankton. 

 The eggs of the skates and most of the dogfishes are large and are 

 provided with a strong capsule or shell. The period of incubation 

 lasts for the greater part of a year and at the end of this time the 

 little fish hatches out and at once settles down to its natural 

 habitat on the sea floor. The empty capsule — the " mermaid's 

 purse " of the children — is by-and-by washed up on the beach by 

 the tides and if one opens it at the right end (the right end is 



