36 LIVERPOOL MARINE BIOLOGY COMMITTEE REPORT. 



which form flattened leaf-like colonies, either growing inde- 

 pendently, or encrusting the surface of rocks or seaweed. In 

 Sponges, the connection between the several individuals of 

 a colony is a more intimate one, and it is usually impossible 

 to determine the boundaries of the several component mem- 

 bers, which are blended indistinguishably with one another, 

 and traversed by a common canal system serving for the 

 nutrition of all alike. 



In other cases, as in the hydroid zoophytes, and to a 

 less extent in the sea-pens and other corals, the several 

 members of the colony, though retaining their essential 

 similarity, become modified in various ways so as to better 

 adapt them to fulfil some one or more functions. As this 

 can only be done at the expense of other functions, a certain 

 amount of mutual dependence is at once set up ; and, in 

 many instances, this differentiation is carried so far that 

 certain individuals of the colony are alone able to digest food, 

 which they do for themselves and for all the others as well. 

 Of these latter, some members are specially modified for 

 capturing prey, others are either actively or passively pro- 

 tective in function, while to others is assigned the formation 

 and ripening of the eggs. 



The structural differences between the several members 

 of such a colony are as marked as the functional, and it is 

 often only by tracing their development that their essential 

 identity can be established. The egg-bearing members of 

 the hydroid colony usually take the form of jelly-fish or 

 medusae, which not unfrequently separate from the colony 

 when ripe, and lead a free swimming existence for a time. 



Of the shallow water fish, many, as the pilchards, white- 

 bait, etc., go about in large shoals ; but perhaps the most 

 interesting are the Pleuronectidse, or flat-fish, such as the 

 turbot, plaice, sole, etc. In these, the body is very much 

 compressed laterally, and the two sides are differently 



