146 THE BIOLOGY OF THE SEA-SHORE 



Not a few of these parasitic Copepods are found asso- 

 ciated with shore animals. Herdman (1892) has noticed 

 the occurrence of a minute species, Lichomolgtis ogilis, in a 

 number of cockles, and considers it to be a commensal. 

 Eliot (1910) reports the occurrence of members of the same 

 genus as common external parasites of Nudibranch molluscs. 

 They occur on a number of species, their coloration varying 

 with that of the host. Copepods are also the most common 

 internal parasites of both Eolids and Dorids. Though they 

 are rightly called internal parasites their egg-sacs may project 

 outside the body of the host. The female of another Cope- 

 pod {Trochicola enterica) described by Dollfus (1914) is 

 found in the rectum of the common shore molluscs Trochiis 

 zizyphinus and Gihhula cineraria. This parasite is elongated 

 and worm-like in form, bright red in colour, and almost 

 fills the rectum, reaching a length of 8 mm. Only one 

 individual occurs in each host. 



Well-known fish parasites of this group are Caligus, 

 Lepeophtheirtis, and others. The species L. pectoralis is 

 most frequent upon Pleuronectes flesiis, the flounder, and the 

 males and immature forms of both sexes are to be found all 

 over the skin on each side of the fish. Mature egg-bearing 

 females, however, are usually situated under the pectoral, 

 pelvic, ventral and dorsal fins, and as many as twenty to thirty 

 may often occur under each pectoral fin. They are affixed 

 by means of their powerful second maxillipeds, assisted by 

 the antennae. These creatures can move rapidly over the 

 skin of the fish if irritated (except mature females which only 

 cling tighter), and they can also swim freely. (Scott, 1901.) 



The remaining shore parasites which we have to consider 

 belong to the worm group, and to the flat- worms in parti- 

 cular. The flat- worms are at once the most highly specialised 

 and the most ubiquitous parasites in the animal kingdom. 

 They illustrate all the most characteristic features of para- 

 sitism including great fertility and complicated life-histories. 

 Very frequently the life of the parasite is linked up with 

 two quite distinct animals, one a vertebrate and the other 

 an invertebrate, the adult parasite occurring in the former. 



