MORPHOLOflY OF MELIBE 541 



1910). The origin of the stomach-plates in M. leonina, 

 which no one seems to have described, is not the same as 

 indicated by EHot (1910: 39-40) for the Dorids. This may be 

 plainly seen in sections which are represented in my drawings 

 (PI. 32, figs. 42, Trs, 43, Trp.pl). In the case of Melibe the 

 plates originate as a secretion-product of the epithelial lining 

 of the gizzard. This secretion accumulates within the cell 

 and is voided, little by little, by the cell into the cavity of the 

 stomach. As these secreted droplets pass into the stomach 

 they become attached to their predecessors, harden, and form 

 into a continuous layer resembling the stratum corneum of the 

 human epidermis but, of course, arises differently. The 

 secretion droplets seem to originate around the nucleus of the 

 epithelial cells ; they then coalesce into larger droplets and 

 finally break up into smaller ones that pass out of the cell 

 and into the stomach and then form by a keratinization (?) 

 process into the hard or protective lining of the stomach. 

 This protective lining, the stomach-plates of Melibe, no 

 doubt serves a double function : to protect the living cells 

 against the spines of the crustaceous food ; to help in mastica- 

 ting the food before it passes into the intestine. This latter 

 use seems to be necessary, particularly in those cases in which 

 the organism is void of mandibles or radula of any kind. 



c. The Pyloric Diverticulum. 



The pyloric diverticulum (PI. 27, fig. 9, Pd,) is described 

 by Alder and Hancock (1845 : p. 14) as pancreatic in function. 

 Whatever function it may have, it seems logical to think that 

 it plays a specific part in the process of digestion because of its 

 internal structure. This part of the alimentary canal is situated 

 at the constricted posterior part of the stomach. Externally it 

 consists of an elaborate evagination into a number of folds, 

 beginning laterally and continuing ventrally, until meeting on 

 the opposite side. Internally (Pis. 32 & 33, figs. 39, 44) the 

 pyloric diverticulum is considerably corrugated, being thrown 

 into much larger folds than the remainder of the tract. Ciliation 

 of the mucous layer begins here. The corrugations are formed. 



