On the Rhizocephalan Genus Thompsonia, etc. 31 



SUMMARY. 



(1) The root system of Thompsonia is continuous throughout the 

 body of the host, whatever the number or stage of development of the 

 external sacs may be. 



(2) Where they penetrate the appendages the roots contain fewer 

 yolk globules and the lacunar space is filled with germ cells. Besides 

 the external sacs, there are a number of terminal swellings in the tissue 

 of the appendages. These are similar in structure to the external 

 sacs and become external at a subsequent moult of the host. 



(3) The external sacs consist of a mantle with an external and in- 

 ternal cuticular investment and a visceral mass mainly occupied by the 

 ovary. No mantle cavity is found between the mantle and visceral 

 mass. 



(4) There is no testis, development being in all probability partheno- 

 genetic. The egg is lightly yolked and gastrulation takes place by 

 epibole. The Nauplius stage is omitted from the life history, the young 

 hatching at the Cypris stage. 



(5) During development the visceral mass disintegrates so that at 

 the time of hatching the mantle contains a great number of Cypris 

 larvae ready to emerge. An apical perforation is made in the mantle, 

 and on the moult of the cuticle a way is thus opened to the exterior for 

 the larvae. 



(6) The escape of the larvae is contemporaneous with or soon fol- 

 lowed by a moult of the host. The empty shells of external sacs are 

 carried away with the cast skin, and the terminal swellings of the root 

 system emerge as a new crop of external sacs. 



(7) Development of the germ cells in the lacunar tissue of the roots 

 may take place in situ as well as in the external sacs, but it does not 

 apparently proceed very far. 



(8) The large number of external sacs in both Thompsonia and 

 Peltogaster socialis is accounted for by a process of internal budding 

 from a single original larva. 



(9) Thompsonia is not a primitive Rhizocephalan, but a very 

 specialised form. 



(10) The genus Thylacoplethus Coutiere is synonymous with Thomp- 

 sonia Kossmann. 



