HO A BIOLOGY OF CRUSTACEA 



females. Both males and females have a characteristic mouth cone 

 which is expanded into a disc. The mandihles make a small hole in 

 the body wall of the host, and blood is sucked out. Many of the 

 species live in the brood pouches of their hosts and attach them- 

 selves to the oostegites. Egg sacs are produced by the females, and 

 are usually freely disposed in the host's brood pouch. The fecundity 

 of each female is remarkable; six or eight egg sacs, each the size 

 of the female, may be produced. Free-swimming larvae hatch from 

 the eggs. These larvae are of a typical copepodan appearance 

 (fig. 48c), but they have the characteristic choniostomatid mouth 

 cone. Some species (e.g. Sphaeronella giardii) go through a pupal 

 stage; this forms from the larva when it finds a new host. It is from 

 the pupa that the young males and females emerge. In some species 

 (e.g. Sphaeronella leuckarti) only the females go through the pupal 

 stage, and in others (e.g. Aspidoecia normani) the pupal stage 

 appears to be missing; the young males and females develop directly 

 from the larvae. 



Rhizorhina ampeliscae is also a parasite on other Crustacea, in 

 this instance on members of the amphipod genus Ampelisca. This 

 is surprising, because Rhizorhina belongs to the family Herpyl- 

 lobiidae, and all the other members of this family are parasites on 

 polychaete worms. The adult female is spherical in shape and has 

 no appendages at all, but she has a remarkable rooting process 

 which branches within the body of the host. This in an interesting 

 parallel with the Rhizocephala (p. 116), although the anatomical 

 origins of the roots are very different in the two groups. The male 

 of Rhizorhina is similar in appearance to the larvae of the Choni- 

 ostomatidae, and is an example of precocious attainment of 

 maturity while still in a larval form. 



With the female Rhizorhina we have arrived at a stage equivalent 

 to a sac feeding by means of roots. Can specialisation and simplifica- 

 tion of structure go further? The answer is yes. Xenocoeloma is a 

 parasite on a marine worm, Polycirrus. The adult of the copepod is 

 merely a mass of tissue lying between the skin and the gut of its 

 host; it has no appendages and has even lost its skin. Another 

 remarkable fact is that it is hermaphrodite; this is a most unusual 

 state for a copepod. The copepodan affinities of Xenocoeloma are 

 revealed by its producing two long egg sacs, and by the nauplius 

 larva which hatches from these eggs. Unfortunately we do not 

 know the stages between the nauplius, which is normal apart from 

 lacking a gut, and the adult. 



