ANIMALS 



45 



The ability of many sessile marine animals to attach to each other, 

 however, tends to reduce the effects of the first arrivals on any sur- 

 face, especially if they are shell bearers. 



The duration of life cycles is of vast importance in determining 

 apparent degree of dominance and the arrangement of species found 

 at any time (Rice, 1935). This is especially true of barnacles and 

 bivalves, which are abundant in the tidal communities and in such 

 subtidal ones as occur in the Danish waters. These short life histories 

 (two to ten years) lead to a rapid replacement of individuals and 

 make the arrangement of subordinate groups and the margins of 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 



Copepod nauplii 



Ascidian larvae 



Balanus nauplii 



Balanus cypres 



Crab zoeae 



Plutei 



Crab megalops 



Fig. 3. — The seasonal sequence of lai\al forms in the plankton of the northeast 

 Atlantic. (After Johnstone.) 



communities shift back and forth rapidly, to a degree unknown on 

 land. 



Parasites. Parasites are of importance in communities only as 

 they decrease the vigor, fecundity, abundance, etc., of dominant or 

 influent organisms. In every case in which they do have essential 

 relations, the determination of the life-cycle characters is of great 

 significance. Generally speaking, four types of parasites may be rec- 

 ognized: (a) bacterial parasites that have the simplest of life histories 

 characterized by very rapid development and decline within the or- 

 ganism and are occasionally responsible for malignant infectious dis- 

 eases of influent animals; (6) internal animal parasites that frequently 

 infect more than one organism and do not ordinarily kill the host 

 appear quite generally to have little or no detrimental effect; (c) 

 external i)arasites, usually chiefly arthrojiods, with very simple life 

 histories which become important occasionally when they are cspe- 



