143 



itself in the great majority of forms as an insignificant appendage turned 

 backwards, or from the mouth, and attached in much the same manner as in 

 other Copepoda. That this foremost pair of oral limbs, together with their 

 palpiform appendage, cannot be regarded as the mandibles, but must represent, 

 as justly opined by Thorell, the true maxilla?, is evident not only from their 

 place far behind the antennal area, but also from their relation to the oral aper- 

 ture. The exact position of the latter is not always easy to determine, because 

 it is more or less completely covered by a lamellar anterior lip; on a closer 

 examination, however, it will invariably be found that the said limbs are not 

 placed, like the mandibles, at the sides of that aperture, but decidedly behind 

 it, turning their extremities more or less forwards, precisely as do the maxilla; 

 in other Copepoda. The armature also of these limbs is very different from 

 that generally met with in the mandibles, and in some cases at least (Clausi- 

 diidae) exhibits an unmistakable resemblance to that found in some of the 

 gnathostomous Cyclopoida, for instance in those belonging to the family 

 Cydopidce. It will finally be noticed that whereas the mandibles in several 

 cases are quite destitute of palps, these appendages are almost invariably found 

 in the maxillae. Thorell describes all the oral limbs as maxillae (m 1 nr); 

 but, according to the usual terminology, the 2 posterior pairs should more 

 properly be named maxillipeds. 



About the parasitic nature of the Copepods belonging to the present 

 section there cannot be any doubt; but in many instances the hosts infested 

 by them have not yet been ascertained. The parasitism is in every case, 

 like that of most of the siphonostomous Cyclopoida, rather incomplete or 

 merely temporary, the animals being able easily to change their place and 

 to move freely through the water. Somewhat more sedentary are of course 

 the habits of such forms as live in the branchial cavity of Ascidians or 

 within the valves of different Mussels; though these forms also may at times 

 be able to leave their hosts. Only a few forms are stated to be true fish- 

 parasites, viz., the species of the genera Bomolochus, Ergasilns and Thersitina; 

 but by far the greater number seem to be confined to invertebrate animals 

 belonging to many different groups e. g. Echinoderms, Worms, Mollusca, Tunicata. 

 A number of forms belonging to this section lead a true pelagic life, being 

 constantly found in the open sea, and probably at times infesting some of 

 the larger pelagic animals. This is the case with all the known forms be- 

 longing to the 3 families Oncceidcv, Corycceidce and Sapphirinidce. 1 ) 



') The parasitism of some species of Sapp/iiritia on Salpoc lias long been known. 



