1. The History and Development of Sacculina interna. AQ 



but this stage is later than that represented in my fig. 6 where not only is there no conti- 

 nuity, but the cells composing the sac are very small like those of the "nucleus" and utterly 

 unlike the large cells of the wall of the central tumour. I think it probable that the sac is 

 hollowed out in situ from the cells forming the rim of the "nucleus", but I do not wish to 

 be dogmatic and the matter is of minor importance. 



By the time the central tumour with its "nucleus" has reached the region round the 

 unpaired diverticulum, the "nucleus" has enlarged, the surrounding sac is more clearly visible 

 and the spider cells in the tumour have increased in number and become more filamentous. 

 The epithelial cells composing the walls of the tumour and of the roots have increased greatly 

 in number and are becoming somewhat smaller. The roots already have a wide distribution. 

 We now pass to the differentiation of the organs of the Sacculina which takes place after the 

 latter has reached its definitive position shown in Plate 5 fig. 1 a. The whole of the Sacculina 

 is of course still entirely inside the body of the crab, the continuity of the abdominal epi- 

 thelium and chitin of the crab being easily demonstrable in serial sections (e.g. Plate 5 fig. 10). 



B. Sacculina interna. Organogeny and Evagination. 



(Plate 5 figs. 9—18.) 



The organogeny of Sacculina interna has been described by Delage and with almost 

 all his description I am in agreement, but with regard to the formation of the mantle and 

 of the perisomatic space I entirely disagree with him. I am surprised that our accounts 

 of the formation of these two organs should be so entirely different, because it is evident 

 that in all other points of anatomy and development Sacculina neglecta and Sacculina 

 carcini are practically identical. But I am not in these respects satisfied with Delage's 

 account, because the figures on which he bases it are to my mind not at all convincing, and 

 one figure, which should be the crucial one, is drawn from a confessedly abnormal specimen 

 (Delage's figure 38). We both agree in starting from the stage figured in Plate 5 fig. 7 of 

 this work, and in Delage's figure 37. We both agree in reaching a stage in which the 

 visceral mass of the developing Sacculina is enveloped by two sacs, an inner one the mantle 

 cavity or brood pouch, and an outer one the perisomatic space, but as to how these two sacs 

 are formed we are in irreconcilable disagreement. 



According to Delage the mantle sac (be) is formed by delamination from the inner 

 wall of the original investing sac (pc), the cavity of this latter sac forming the cavity of the 

 perisomatic space. This view is presented in the Diagrams in Text fig. 12. 



According to me the original investing sac (Plate 5, be in figs. 6 and 7) remains un- 

 altered as the mantle cavity or brood pouch, while the perisomatic cavity is formed by a 

 separate invagination of the wall of the central tumour. This view is presented in the 

 Diagrams Text fig. 1 3. 



Zool. Station zu Neapel, Fauna und Flora, Golf von Neapel. Rbizocephala. 7 



