ON POLYZOA AND SHELL-FISHES. 17 



brief, its circumstances are different from those, for instance, 

 of a fish with pelagic eggs. 



In this group, therefore, we see that, while the more minute 

 and lower forms are beyond the interference of man, certain of 

 the larger and higher species can be notably diminished. So 

 far as can be ascertained, however, the species found on the 

 bottom of the open sea are as abundant as in former ages, and 

 constitute an important element in the nourishment of the 

 ground -feeding fishes, such as the cod. 



The wealth of oceanic life receives a considerable increment 

 from the Polyzoa, which are either tufted plant-like forms or 

 calcareous crusts on stones and sea-weeds. The group, how- 

 ever, is not a conspicuous one in the pelagic fauna, though the 

 larvae certainly occur in abundance, and for many months of 

 the year. The sea-mats (Flustrce) are especially plentiful on 

 some fishing-grounds, and, besides the larvae which increase the 

 pelagic life, the arborescent tufts give shelter to other forms on 

 which fishes feed. 



In the offshore as well as the inshore areas, in shallows and 

 in very great depths, at the surface, in mid-water and on the 

 bottom, the group of the shell-fishes is everywhere disseminated. 

 The vast resources of the sea in this respect cannot be over- 

 estimated. For the present purpose the class may be divided 

 into two groups, viz., (1) the pelagic, and (2) those frequenting 

 the bottom. 



The pteropods, heteropods, and the pelagic stages of 

 demersal forms — both univalve and bivalve — are universally 

 distributed from the Arctic to the Antarctic seas. Thus Clione 

 and Limacina form conspicuous elements of the pelagic food of 

 the right whale in the Arctic seas, while the tropical and 

 sub-tropical pteropods are even more numerous. That these 

 and their allies are a favourite food of fishes is well known, 

 though it is less generally understood that even ducks feed on 

 them in the surface-w^ater. Mixed with the purely pelagic 

 mollusks are immense numbers of the larval stages (veligers) 

 of bivalves and univalves, besides sea-slugs and their allies, 

 and they are found during every month of the year, most 

 numerously in spring and summer. The neighbourhood of a 

 M, R. 2 



