COMMENSALISM AND PARASITISM. 169 



forms which may take the opportunity to attach 

 themselves as they pass through the meshes of 

 the gill filaments. Moreover, it is in the gills 

 particularly that the blood current comes into 

 closest contact with the water, and it requires 

 but a little puncture on the part of the young 

 parasite to reach a constant supply of this nour- 

 ishing fluid. 



It is in the gill chambers that we find most 

 frequently representatives of that interesting 

 group of animals, the parasitic Cope- 

 pods. 



It would be difficult to recognise 

 them as Copepods if we were to 

 judge by their adult characters alone. 

 Unlike the brisk, brightly- coloured 

 creatures with long rowing antennae 

 that we have described above as 

 living a free life in the surface waters 

 of the ocean, these parasites have a 

 white sac-like body, with short blunt 

 processes representing the legs, no FIG. 41. A 

 eye, and generally two long thread- g* g a s d itic 

 like bags of eggs attached to the sides 

 of the rudimentary tail. As we found in the 

 case of the Sacculina, the true zoological position 

 of these parasites can only be determined by 

 reference to their developmental history. 



In the gill chamber of the Prawns we find a 

 very much modified parasite, which is closely 

 allied to those skin parasites of Fishes mentioned 

 above. Many of my readers may have noticed 

 that in some Prawns there is a wart-like swelling 

 on one side of the neck. If the skin be removed 



