On the Structure and Affinities of Mnestra parasites Krohn etc. 45 



the axial line of the medusa, so that there the ectoderm comes into 

 contact with the eudoderm of the gastric cavity. 



In this respect Mtiestra resembles such forms as Eleutheria in 

 which a "Scheitelhöhle" or exumbral cavity is developed. 



Endoderm. 



The endoderm cells of the orai end of the manubrium bave 

 lost the usuai epithelial structure, and as has already been men- 

 tioncd, form a meshwork of spongy reticulated tissue which entirely 

 fills the inside of the manubrium, as shown in the transverse sec- 

 tion (fig. 19). In optical section this plug- of reticular tissue strongly 

 resembles the open meshwork of digestive cells which has been seen 

 by many in other Coelenterata, e. g. Lankester and Günther in 

 Linmocodiwn; but whereas in the latter case the endoderm is a 

 mere inner lining to the manubrial canal, it forms a plug completely 

 tìlling the cavity in Mnestra. 



Spherical nuclei like those in the other gastric endoderm are si- 

 tuated at intervals in the Strands of protoplasm which connect the 

 cells with one another. Within the intercellular spaces of this cell- 

 complex may occasionally be observed small round corpuscles 

 indistinguishable from small round corpuscles in the blood of the 

 Phyllirìioe and also in the other gastro-vascular spaces of the 

 Mnestra. 



It is worthy of notice that in Mnestra cells seemingly devoted to 

 digestiou occur in the distai portion of the manubrium, whereas for 

 the most part, in other medusae, such digestive cells line the proxi- 

 mal portion of the digestive cavity alone. This difference may be 

 attributed to the difference in the food of free living and of para- 

 si tic medusae. 



The rather irregularly shaped centrai gastric cavity is lined 

 with a protoplasmic network containing many nuclei (lig. 20). The 

 intercellular spaces in this region are much smaller and the proto- 

 plasmic Strands are much thicker than in the manubrium. 



Endoderm of radiai and eireular canals. 



The endoderm at the junction of the radiai canals with the 

 stomach is of the nature of a ciliated pavement epithelium (tig. 21). 

 The cells bave spherical nuclei, irregulär polygonal outlines and 



