I 1 8 A BIOLOGY OF CRUSTACEA 



the size of the body, and are joined along the whole length of 

 their edges apart from a small opening near the mouth. The para- 

 site feeds by sucking fluid from the tissues of the host through a 

 conical structure around the mouth. Branches of the gut and ovary 

 ramify throughout the enlarged carapace. This branching of the 

 internal organs is important, because it foreshadows the develop- 

 ments found in more highly modified members of the same group. 



In Myriocladus and Dendrogaster, which are parasites of echino- 

 derms, the outgrowths from the gut and ovary extend much 

 further than the confines of the carapace, giving the animal a bush- 

 like appearance. The small body lies at the centre of the ramifica- 

 tions, and houses the mouth, through which fluids are sucked from 

 the host. 



The males of all Ascothoracica are dwarfs and are to be found in 

 the mantle cavity of the female; in general the males are much 

 less modified than the females and are quite recognisable as 

 modified cypris larvae. 



It is a remarkable feature of parasitic Crustacea that no matter 

 how bizarre and distorted the adult may be the larvae remain true 

 to type. The persistence of a nauplius in Sacculina and of a cypris 

 in Thompsonia illustrate this. A further point is that the males are 

 often dwarfs, which retain larval characteristics. The cypris-like 

 males of the Ascothoracica have just been referred to, and among 

 the copepods the male of Rhizorhina is remarkably like the larvae 

 of the Choniostomatidae. The precocious attainment of sexual 

 maturity while still in a larval stage is a feature we shall meet again 

 in the parasitic isopods, some of which provide a partial explanation 

 of the phenomenon. 



PARASITIC ISOPODS 



All the members of one sub order of the isopods, the Epicaridea, 

 are parasitic on other Crustacea. Their life histories are often com- 

 plicated and involve two hosts. The eggs give rise to larval forms 

 which are essentially typical little isopods, except that they have 

 hooked claws on the legs, and the mouthparts form piercing stylets. 

 This is known as the epicaridian stage, and it swims actively in the 

 sea until it finds a copepod, such as Calanus. The epicaridium 

 attaches to the copepod and sucks its blood. It remains on the 

 copepod for six or seven days and passes through a couple of moults. 



