374 



THE INVERTEBRATA 



tive custom of feeding by the automatic straining of food particles 

 from the water is retained : the feeding current eddies from the swim- 

 ming current which the antennae, mandibles, and maxillae set up, 

 and is strained through a fringe of bristles on the maxillae (Fig. 251). 

 The parasitic habit has been adopted by members of very different 

 families of copepods, and to very various degrees even by members 

 of a single family. Every stage may be found between normal, free- 

 living forms and the most degenerate parasites. Parasitic forms often 

 have a suctorial proboscis, which is formed by the upper and lower 

 lips enclosing mandibles adapted to piercing. Such a proboscis is not 

 necessarily accompanied by a high degree of degeneration. The life 

 histories of parasites are often complicated, and may involve remark- 

 able changes of habit. Degenerate forms usually reach one of the 



ex- 



Fig. 251. The maxilla of Calanus. ex. small prominence which perhaps re- 

 presents the exopodite; i and 2, endites representing the first two segments; 

 9, terminal segment. 



Cyclops Stages and may pass through them all before they begin to 

 degrade. Often the male is less degenerate than the female: he may 

 be free-swimming while she is sedentary, or may be much smaller 

 and cling to her body. It is only possible here to mention a few of the 

 numerous genera of these interesting parasites. 



Notodelphys, commensal in the pharynx of ascidians, is clumsy 

 bodied, and has a large dorsal egg pouch on the 5th and 6th thoracic 

 somites, but can swim and is sometimes captured outside the host. 



Monstrilla has a very remarkable life history. The adults of both 

 sexes are free-swimming, as are the newly-hatched Nauplii^ but the 

 intermediate stages are parasitic in various polychaets, where they 

 absorb nourishment by means of a pair of long, flexible processes 

 which represent the antennae. In this stage they lay up a food supply 

 for the entire life cycle, throughout which the animals are without 

 functional mouth parts or alimentary canal. 



