On a Parasitic Copepod from Cephalodiscus. 181 



Plate XVIII., fig. 1, but it appears to be a matter of indifference 

 whether the head or the tail is directed towards the anterior end 

 of the stomach. 



The adult females are most common, and in every case only 

 a single one of these occurs in one host. The eggs were never 

 observed attached to the body, but isolated eggs, or packets of eggs, 

 were sometimes found free in the host's stomach. The number 

 of eggs deposited at one time appears to vary somewhat. Two 

 packets of two each, two packets of three, and from three to five 

 isolated eggs were observed on different occasions. The diameter 

 of an egg is about *12 mm. Of the larval stages, two, three, or four 

 specimens occurred together in one stomach, accompanied or not 

 by an adult female. No adult male was found, and from the 

 analogy of Enterognathus we may assume that this stage is free- 

 swimming. 



In several cases it was observed that zooids containing specimens 

 of the parasite had well developed and apparently ripe gonads, 

 so that there would appear to be no " parasitic castration" of 

 the host. 



It is not altogether easy to reconstruct from our observations the 

 probable life-history of the parasite. From the relatively large size 

 of the eggs it may be assumed that hatching takes place at a late 

 stage of development. On two occasions specimens in a stage 

 corresponding to that shown on Plate XVIII., fig. 9, were found in 

 the act of escaping from a membranous investment which appeared 

 to present no trace of appendages, and was at first regarded as the 

 egg-membrane. The fact that from two to four of these young 

 individuals always occur together seemed also to suggest that they 

 were hatched in situ and had to pass through a free-swimming stage 

 before entering each a separate zooid of the host to become adult 

 females. Further search, however, afforded no evidence of any 

 break in the series of forms, all of them plainly incapable of 

 locomotion, connecting these young stages with the adult. As 

 it seems necessary to assume the existence of a free-swimming 

 stage at some period of the female's life-history (the male is 

 probably, as already stated, free-swimming when adult) there can 

 be little doubt that the earliest stages observed are preceded, as 

 in Enterognathus, by a free-swimming stage in which the larvae 

 pass from one host to another. As in Enterognathus also, the 

 females probably emerge temporarily from their hosts when sexual 

 maturity is reached in order to be fertilised by the free-swimming 

 males. On again becoming endoparasitic they each select a host 

 which does not already harbour a parasite. 



