266 



uiscovi;uY 



exception <if the liirring, all our food fish of economic 

 importance lay eggs which are lij;litcr than sea-water 

 and which, therefore, float upwards to the surface 



lavi r<. wluic they arc found in grnit .iliuiidincc in 



—FOOD or MACKIiRIil, 



the spring months of the year. The two great families 

 of economic value, the Cod family {Gadidce) and the 

 Plaice family (PleuronectidcE) , arc represented in the 

 surface waters of the British Isles by many millions of 

 eggs and larvae every spring. These eggs may be 

 captured by towing a small-meshed net through the 

 water, a method in which muslin or silk bolting cloth 

 is the material generally used. If the contents of the 

 net be washed out and examined with a hand lens, 

 numerous globules of gelatinous matter are readily 

 distinguished. These are the floating fish-eggs. Many 

 hundreds are seen in a typical haul, but it is impos- 

 sible in most cases to recognise the species without 

 careful examination under the microscope. 



The egg of the plaice may usually be recognised by 

 its large size. The cod and whiting have smaller 

 eggs, and, generally speaking, they are surrounded by 

 numbers of a small cope-pod crustacean (Calanus). 

 Later in life our food fish belong to the Nekton, 

 since they are able to move independently of 

 the currents. Economically speaking, Crustacea form 

 one of the most important groups in the plankton, 

 if we exclude fish. Some of the Crustacea are 

 represented in both their young and adult stages, 

 others in the larval condition only. The edible crab, 

 which passes its adult stage on the bottom (Benthos), 

 passes through a free-swimming life-history in which 

 two main stages have been recognised ; in both of these 

 stages it belongs to the plankton. 



The first larval stage of the crab is the Zcta. in which 

 the carapace, or upper shell-covering, bears a I 

 spine. This is followed by a stage in which the 1' 

 gradually assumes a crab-like appearance, though i in- 

 tail is still extended, as in the lobster, and not curled 

 up under the carapace. This is known as the 

 Megalopa stage. 



Even the fixed Crustacea, such as the common bar- 

 nacle, fKiss through a free-swimming larval stage, 

 in which they belong to the plankton. The larvae 

 of the barnacle are distinguished by three pairs of 

 swimming appendages and are known as Nauplii. 



Our edible mollusca, such as the oyster, cockle, and 

 mussel, appear in the plankton only in the larval 

 stage. Other mollusca, such as the Heteropoda and 

 Ptcropoda, spend the whole of their life as plankton 

 organisms. 



Marine worms, generally speaking, are only found in 

 the plankton during their larval stages, but there 

 exist a few forms w^hich are found only in the open 

 sea, such as the arrow-worm (Sagilla). Jelly-fish, or 

 Medusa, are also members of the group. Most of 

 the common jelly-fish are large and easily recognisable, 

 but there are several forms which are small enough to 

 be taken in the tow net. Two of the commonest of these 

 small forms in British seas are knowTi as Pleurohrachia 

 and Beroe. The former is known to the fishermen 

 as the " Marble-bleb," and is often taken in the 

 shrimp trawl. 



All the forms hitherto mentioned definitely belong 

 to the animal world. Other constituents of the 

 plankton belong no less clearly to the vegetable 

 world, while others still are of doubtful position. 

 So numerous are the plankton organisms that it is 

 no exaggeration to say that every drop of sea-water 

 near the surface contains some form of life. Countless 

 multitudes of unicellular forms of life are encountered. 

 The most important animal forms belong to the 

 Infusoria, Foraminijera, and Radiolaria. Of the 

 plants the most important group is that of 

 the Diatoms^ Iniusoria are more common in fresh- 



water plankton than in the sea. Nevertheless, one of 

 the commonest of local organisms belongs to the 

 Pcridinians, which are Infusoria provided with shells 



