PLANT AND ANIMAL ASSOCIATIONS 57 



find concealment and nourishment among them were able 

 to colonise the canal. So far, however, no Fucus or Zostera 

 was present in the canal, and no Palcemon or Nerophis. The 

 sea-urchin occurred, however, and in the absence of Zostera 

 fed on Enter omorpha. Species of Idothea, Rissoa^ and Lit- 

 torina were present in small numbers. A few mud-dwellers 

 had put in an appearance in addition to several worms. 

 The brittle-star Ophioglypha entered the canal some eighteen 

 months after the opening. Many mud-worms, mud-fre- 

 quenting Nematodes, and forms such as Scrobiciilaria, My a 

 truncata, Acer a hullata, Molgula, etc., were still absent in 

 the autumn of 1896 and could not appear in any numbers 

 until a more copious deposit of mud had been formed by the 

 decomposition of the algae growing on the banks. At that 

 stage a slight mud deposit had already accumulated from 

 the remains of dead planktonic organisms. 



Sorby (1901), in the course of ten to twelve years' collect- 

 ing along the coasts of Essex and parts of Suffolk, has noticed 

 that while certain forms appear to be more or less constant, 

 many others show very considerable changes both in their 

 distribution and frequency of occurrence. Various factors 

 are no doubt concerned, such as extremes of temperature 

 (particularly cold), cyclical weather changes influencing 

 salinity ; human agency operating through the discharge 

 of sewage in estuaries, the dilution or contamination of 

 estuarine water resulting from the drainage of fields, the 

 taking of fish, shrimping, and the various operations of 

 oyster culture, etc. Sorby claims to know of a case in which 

 the largely increased amount of fresh water entering an 

 estuary at a time of heavy rains killed off thousands of pounds 

 worth of oysters. 



A mussel bed illustrates vividly the competition within 

 a species, i.e. between its individual members. These are 

 frequently so numerous as to occur growing upon one 

 another until they are several layers deep and actually smother- 

 ing and poisoning one another by the accumulation of waste 

 products and the silt, etc., that settles among their masses. 

 The manner in which the various members of the shore 



