16 



Papers from the Department of Marine Biology. 



the root system has had time to spread throughout the whole body 

 of the host. From the fact that the external sacs appear at first only 

 on abdominal and never on thoracic appendages alone, we may safely 

 conclude that the internal stage of Thompsonia fixes and grows in the 

 abdomen of the host, as is the case in Sacculina and probably the other 

 members of the Rhizocephala. 



THE STRUCTURE OF THE EXTERNAL SACS. 



It has already been pointed out that the external sacs are rounded or 

 cylindrical bodies attached to the host by a peduncle and varying in 

 length from 0.5 to 1.0 mm. in the parasite of Synalpheus, attaining 

 to 3 mm. in that of Thalamita. The smaller forms are immature, 

 the larger are occupied by larvae ready to hatch. The external sacs 

 are probably homologous with those of the typical Rhizocephala, but 

 differ from this in the simplicity of their structure, a simplicity which is 

 due to their large number and small size. Of the organs contained in 

 the visceral mass of Sacculina or Peltogaster, the nerve ganglion, repro- 

 ductive ducts, muscular tissue, and probably too the testis, have been 

 lost. 



ch.ex. 



-m. 

 -ch.in. 



--o 



FIG. 7. Young external sac of Thompsonia; whole preparation to show general 

 appearance of mantle and ovary. X60. 



F IG . g. Portion of transverse section of a more advanced external sac. X240. 

 in., mantle; ch. ex., external layer of chitin; ch. in., internal layer; vac., 

 vacuolated external layer of visceral mass; o., ovary, with ov., develop- 

 ing eggs, and i.e., interstitial cells. 



Coutiere states that the external sacs of Thylacoplethus show the 

 following typical Rhizocephalan structures: "Tin manteau a double 

 paroi, dans lequel est suspendue une masse viscerate et qui porte une 

 ouverture cloacale." With this general summary I am in agreement. 

 Hafele, on the other hand, does not appear to recognise any homology 

 between the external layer of tissue (aussere Gewebeschicht) and the 

 mantle of other Rhizocephala, or between the internal body (internale 

 Gewebeschicht and Ovarium) and the visceral mass. There is, however, 



