1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 839 



eggs enclosed in their egg-shells are extrnded ; the copulatory or 

 vaginal aperture {rag.ap.), through which the sperm of the male 

 is received ; and a third minute n.-pertnre {ap.de. ) situated further 

 back, whose function I cannot determine : I shall refer to it as 

 the apertui-e of the ductus enigmatic^is. This third aperture is 

 not present in jVeritina Jiuviatilis nor in the mai'ine species of 

 Nerita, but is present in all the fresh- or brackish- water tropical 

 species hitherto classed as Xeritina. To begin with a description 

 of the diagram, fig. 3. The ovipository aperture leads into avast 

 pouch with thick glandular walls. This has been called the 

 "uterus " by Gilson and Lenssen, the "shell-gland " by Thiele. As- 

 there is no doubt that the egg-shell is foi'mecl from the secretion 

 of the glands opening into this cavity the latter name is appro- 

 priate, but it will avoid confusion if I bori'ow a name from an 

 analogous structure in the Platyhelmia and call it the " ootype." 



A flattened saccular diverticulum {crij.s.), lying on the right 

 side of the terminal part of the ootype, opens into the latter not 

 far from the ovipository aperture. In the species I examined it 

 is of small size and does not contain concretions or foreign 

 bodies, but it is clearly homologous with the " poche a cristaux '' 

 of Lenssen, and I shall refer to it as the crystal-sac. In Xerita 

 and Xeritina it is relatively large and filled with spherical 

 crystalline bodies. Thiele calls this sac the uterus, but the name 

 is inappropriate, for there is no evidence wdiatever that the ova 

 are passed into it. 



The I'elations of the ootype may be studied in the series of 

 transverse sections (figs. 6-12). Anteriorly it lies above and 

 partly embraces the rectum. In this region its walls are very 

 thick and glandular, its cavity large, crescentic, and simple- 

 Further back it lies to the right of and apart from the rectum, 

 and its cavity becomes more complicated in form. The whole- 

 organ is spirally twisted, so that the concavity of the crescent, 

 which was at first directed dow-nwards, is in fig. 9 directed 

 upwards. In fig. 10 the left-hand corner of the cavity is seen to 

 be j)rolonged into a diverticiilum, which in fig. 11 turns sharply 

 to the right and runs back jjarallel to its former course. From 

 this, which may be described as the terminal part or fundus of the 

 ootype, two passages are given ofi' in difiierent directions. The 

 one, which I shall call the egg-duct, leads to the oviduct and 

 through this to the ovary. The other establishes a connection 

 with the vagina and ductus enigma,ticus and serves for the admis- 

 sion of spermatozoa to the fundus of the ootype. 



The egg-duct {eg.d. in figs. 10-13) is characterized by the 

 difierent forms of glandular tissue constituting its walls. Its 

 opening into the cavity of the ootype is embraced by a glandular 

 thickening, distinguishable from the fact that its cell-contents do 

 not stain in any of the ordinary dyes : this I call the clear gland. 

 As shown in figs. 11, 12, 13, it is largely situated in a tongue- 

 sha,ped projection of the right-hand wall of the fundus, which 

 appears to function as a, valve guarding the passage from the 



