1. The History and Development of Sacculina interna. 



53 



R 



m£S- 



'/•:"' -:^iL et 



Now the long axis of the mesentery, and in consequence the morphological long axis 

 of the Sacculina body, coincides with the morphological long axis of the crab, the organs of 

 the Sacculina being situated symmetrically on either side of this long axis; in other words 

 the symmetry of the Sacculina corresponds to the symmetry of the crab. 



The nervous ganglion (gn) however is formed later in an asymmetrical position, namely 

 slightly to the right of the middle line and just anteriorly to the extreme anterior end of the 

 mesentery. 



On general grounds, explained in Chapter 1 p. 10, I regard the mesenterial surface 

 of the Sacculina as the dorsal, so that the nervous ganglion is formed apically and slightly 

 laterally. 



The main arm of the perisomatic in- 

 vagination (pc) is pushed up between the 

 visceral mass of the Sacculina and the ven- 

 tral surface of the crab's intestine. 



To summarise the morphological 

 position of the Sacculina during the early 

 stages of organogeny we may say that it is 

 applied by its dorsal or mesenterial surface 

 to the ventral surface of the host's intestine, 

 the bilateral symmetry of its organs coinciding 

 with that of the crab. 



Now during the formation of the pe- 

 risomatic space a rotation of the body takes 

 place, of such a kind that the mesenterial 

 surface of the Sacculina is pulled towards 

 the morphological right side of the crab's 

 intestine, forsaking its median symmetrical 

 position. 



In this manner when the Sacculina 

 interna becomes external, its position on the 

 crab is asymmetrical, the mesentery being 

 situated on the morphologically right side 

 of the crab's abdomen, and the piane of symmetry of the Sacculina being horizontal to that 

 of the crab. 



I have explained in Chapter 1 how the asymmetry of Sacculina relatively to its host 

 can be derived from the symmetrical condition of Peltogaster, and it may be now pointed out 

 that the passage of Sacculina through an embryonic stage in which it is symmetrically applied 

 to its host along its dorsal or mesenterial surface, as is the case in the adult Peltogaster, is 

 strikingly confirmatory of the views developed in Chapter 1. 



me 



Text fig. 14. 



et = centrai tumour. me = mantle-cavity. 

 pe = perisomatic cavity. gn = 



mes = mesentery. 

 "ion. 



