10 Papers from the Department of Marine Biology. 



can well understand why Coutiere and Hafele were unsuccessful in 

 tracing the course of the system. 



Coutiere was nevertheless able to make out some details of their 

 distribution. His description may be given in full: 



"Celles-ci s'ont bien differentes de ce que 1'on remarque chez Sacculina 

 carcini par exemple; elles sont meme plus reduites que dans le genre Sylon, ou 

 Hoeck les a decrites. Ces racines occupent uniquement le bourrelet trans- 

 versal du pleosternites dont elles ont vraisemblablement provoque la forma- 

 tion anomale; on n'en trouve nulle trace autour de 1'intestin ni entre les 

 muscles. Elles envahissent, par contre, les lames concentriques du neurileme 

 externe, tres epaissi chez A. edwardsi et A. avarus. Elles ont un diametre de 

 25 fj. a 30 ju et se montrent ramifiees et coutournees en tous sens." 



Though Coutiere is mistaken in supposing that the roots are local- 

 ised in the neighbourhood of the sternum, his observations as to their 

 absence round the intestine and concentration round the nerve cord 

 agree with mine. 



In Pilumnus, Hafele had a particularly unfavourable host for study- 

 ing Thompsonia. The external sacs are all situated on appendages 

 with an enormously thick cuticle. The only possible method of study- 

 ing the root system was by prolonged treatment of the appendage in 

 Perenyi's fluid to soften the chitin, after which sections could be cut. 

 From an examination of these he concluded that there was a root 

 system of an exceedingly simple kind. Kriiger from a more careful 

 examination of material from the same source, overstaining his sections 

 and washing out carefully, was able to demonstrate a root system of a 

 normal kind and show that the chromatin-rich nuclei which Hafele 

 had taken to belong to the root system were actually in the connective 

 tissue and blood cells of the host. The root system, then, Kriiger 

 decides, does not support the claim that Thompsonia is primitive, and, 

 on the other hand, the omission of the Nauplius stage is a mark of 

 specialisation. This is perfectly correct, and if he had suspected that 

 the roots of adjacent sacs were continuous Kriiger would have been 

 able to complete his chain of reasoning. 



My own conclusion is that there is a single root system continuous 

 throughout the host, from which all external sacs are budded off so that 

 each host is parasitised by a single individual and not by a hundred or 

 more gregariously inclined Rhizocephalans. 



I was first able to see the root system in the abdominal appendages of 

 an infected Synalpheus. Those which bore external sacs were cut off 

 from the living animal and examined under a low power of the micro- 

 scope. The endopodites and exopodites are greatly flattened and the 

 cuticle is thin and unpigmented. The whole organ is thus transparent 

 and the roots are visible as slender strands to which the presence of 

 innumerable highly refringent yolk granules gives a greyish colour. 

 Usually a single root strand entered each ramus and this gave off a 



