VI CRUSTACEA 263 



" winter eggs " which are larger, richer in yolkj and devoid of the capacity 

 of developing without fertilization. The fertilized winter eggs after 

 going through the early stages of segmentation become dormant and 

 remain so throughout the period of unfavourable conditions. When 

 ecdysis takes place they remain within the shed cuticle which round the 

 brood-space becomes much thickened and dark in colour. The rest of 

 the shed cuticle breaks away and the two winter eggs are left within a 

 sort of protective case (the ephippium) formed by the thickened and 

 dark-coloured portion of the cuticle. These egg-cases float about on the 

 surface, or lie in the mud if the water dries up, and when conditions 

 again become favourable — i.e. in the spring in cold climates — each egg 

 hatches out as a female. These females give rise parthenogenetically to 

 other parthenogenetic females and it is only after several generations 

 that occasional males begin to make their appearance. 



The CopEPODA are represented typically by the genus Cyclops (see 

 Fig. 94, p. 204) a common inhabitant of fresh water. They have 

 normally a pear-shaped body, without carapace or radiate eyes, and the 

 developing eggs are carried about by the female in the form of rounded 

 masses, usually one on each side, attached to the abdomen. 



The young Copepod passes through the well-defined type of larva 

 known as a nauplius — a short-bodied larva (cf. Fig. no) possessing three 

 pairs of appendages by the movements of which it swims. These larval 

 appendages persist as the first three pairs of appendages of the adult — 

 first antennae, second antennae, and mandibles — so that we may regard 

 the nauplius as the precociously developed head region of the individual, 

 the remainder of the body being added on during the later stages of 

 development. 



The genus Cyclops has already been mentioned as serving as host 

 to the young larva of Dracunculus (p. 204). The group Copepoda 

 includes a large number of other genera inhabiting the sea or fresh water 

 and constituting a large proportion of the plankton or drifting fauna. 

 They provide an important food-supply for fish and they play a large 

 part in producing the " phosphorescence " of the sea, the actual produc- 

 tion of light in such cases being apparently due to some obscure chemical 

 reaction between the sea water and secreted material produced by the skin. 



Various Copepods have taken to a parasitic mode of life, infesting 

 especially the gills and skin of fish. Some of these have become extra- 

 ordinarily modified in structure so as to be quite unrecognizable from their 

 general appearance as Copepods or even as Crustaceans. Even when the 

 adult is modified in this way, however, a clue to the relationships of the 

 creature is afforded by its young stages, there being a typical nauplius larva. 



