On the Rhizocephalan Genus Thompsonia, etc. 9 



Decapod Crustacea respectively, although, owing to the scantiness 

 of my material, I was unable to investigate thoroughly those of the 

 latter class. The account which follows is based almost entirely on 

 the parasite of Synalpheus. 



THE ROOT SYSTEM. 



The root system is the characteristic organ of the Rhizocephala. 

 Delage and Geoffrey Smith have shown that it appears first in the 

 course of endoparasitic development and is well established before the 

 visceral mass begins to form. There is no doubt as to its existence 

 and importance in all genera, with the exception of the doubtful form 

 Duplorbis, but it has been exactly described only in Sacculina. The 

 following brief description is condensed from the account of Delage: 



The tissue of the peduncle is bounded by the basilar membrane and from 

 this the roots originate. For the most part they originate in a small number of 

 large trunks which immediately ramify and spread. At first the roots follow 

 the intestine, which they envelop with a close network. Some then pass into 

 the distal part of the abdomen of the crab, penetrate the muscular layers of the 

 body wall, and enter the abdominal appendages. Others pass forward and 

 divide into two groups: the first follows the intestine and passes on to the 

 stomach and into the interstices of the liver lobes and the genital glands; 

 the second, not quite so important, accompanies the nerve chain, its branches 

 following the lateral nerves into the limb muscles and penetrating into all 

 the appendages, limbs, antennae, and even the peduncles of the eyes. All the 

 organs are invaded except the gills and heart. 



The roots branch with an irregular dichotomy. The separate branches 

 never anastomose. In diameter they usually vary from 30 to 40/x, but the 

 largest may measure 200ju. Histologically they consist of the following ele- 

 ments: (1) an exceedingly thin external layer of chitin; (2) a layer of cells 

 forming a complete internal investment; (3) the interior of the root is occupied 

 by star-shaped cells, the processes of which anastomose with each other and 

 with the marginal cells. In the largest roots the centre is empty. Refringent 

 drops or granules of yolk occupy the meshes. The root system of Peltogaster, 

 though similar to that of Sacculina in details of histology and method of 

 branching, has a very different distribution. It constitutes a compact mass 

 Avhich is concentrated in the neighbourhood of the peduncle, not invading the 

 spaces between the tissues of the host in the manner characteristic of Sacculina. 



Such accounts of the root system of Thompsonia as have been given 

 by the earlier observers are conflicting and obviously influenced by a 

 predisposition to accept the theory that each external sac is a meta- 

 morphosed larva. It is, moreover, exceedingly difficult to demonstrate 

 the root system of Rhizocephala when preserved by usual methods, for 

 the roots, owing to the impenetrability of their cuticular envelope, take 

 up stains much less readily than the surrounding tissues. In addition, 

 the highly refringent yolk globules, which make the roots easily dis- 

 tinguished in life, disappear under the action of alcohol or even formalin. 

 And lastly, the roots are even more delicate than in Sacculina, so that we 



