128 MARKETABLE BRITISH MARINE FISHES CHAP. 



larva of this kind would be about inch. It is remarkable for 

 the length of the spines which project forwards and backwards 

 from the head region, and for the length and size of the tail, 

 which in the fully developed crab is rudimentary. Fig. 77 repre- 

 sents a larva of one of the tube-inhabiting worms ; among the 

 features by which this form is distinguished is the great length 

 of its bristles. Fig. 78 shows the structure and appearance of a 

 mollusc larva ; in this case the larva of one of the shell-less or 

 nudibranchiate forms. The larva however is provided with a 

 shell into which it can withdraw itself, and with an operculum 

 with which it can close the aperture. It sustains and propels 

 itself -by means of the long vibratory processes or cilia on the 

 edge of the lobes which protrude from the shell. In Fig. 

 79 is represented the larva of one of the echinoderm class, 

 namely a sea-urchin. One chief peculiarity of this larva is 

 the presence of eight very long arm-like projections, each of 

 which is supported by a calcified rod. It will be understood 

 that to the unaided vision such creatures as those figured 

 appear in a glass vessel of sea- water merely as moving 

 specks, and that the figures represent them as seen under 

 the microscope. 



A complete account of the animals which serve as food for 

 fishes would embrace nearly the whole of marine zoology, and 

 indeed a thorough knowledge of the subject including the par- 

 ticular forms on which each kind of fish depends in its early 

 stages, and in the different regions . it visits, and the mutual 

 dependence of the various animals in the sea is not yet available, 

 but has to be obtained by further investigation of the most 

 minute and extensive kind. One more interesting form however 

 may be mentioned here. 



Its shape and appearance are represented in Fig. 82. It 

 belongs to the same division of Crustacea as the opossum shrimps, 

 which differ from the common shrimp in having usually eight 

 pairs of long anterior (thoracic) legs, each of which forks into 

 two branches. This particular species and its nearest relations 

 have the remarkable power of giving out light, a property which 

 not belong to the whole body but is localised in ten special 

 rounded organs, four in a single row on the abdomen, two pairs 

 on the bases of the anterior limbs, and one on each eve-stalk. 



