1908. J ASPIDOBRANCH GASTROPOD MOLLUSCS. 875 



there are half a dozen or more sperm atophores in the sac, but 

 sometimes only one. In no case have I found an empty sperma- 

 tophore-sac. Though I have tried in manj^ difterent ways, I have 

 not been able to make any preparations giving a satisfactory 

 demonstration of the structure of the spermatophores. They are 

 brittle, and are always contracted and distorted by the action of 

 reagents ; probably the study of fresh specimens is necessary for 

 the elucidation of their mechanism. As shown in figs. 66 and 69, 

 the cylindrical body contains a central cavity filled by a mass of 

 spermatozoa. The wall of this cavity is formed of a thin layer 

 of a hard brittle substance which must be of the nature of chitin. 

 Around it is a protoplasmic layer (fig. 66, III.), from which a 

 number of fine filaments radiate to an external wall composed of 

 an elastic homogeneous substance. The radiating fibres pass 

 from the inner to the outer walls at regular intervals, so that the 

 body of the spei-matophore appears in a side view to be made up 

 of a number of segments. The layer of protoplasm surrounding 

 the central capsule is filled with chromophilous granules, but 

 there is no trace of nuclei. Both the inner capsule and the outer 

 wall are continued into the filament, which is therefore a double 

 tube. There is some evidence that the filament is coiled up 

 within the capsule and aftei'wards shot out much as is the filament 

 of a nematocyst, but of this I cannot be certain. It is a curious 

 thing, of which I can ofi'er no explanation, that neither I nor any 

 •of my predecessors have seen any trace of a spermatophore in the 

 male organs. It seems certain that they must be formed in the 

 terminal sac, but there is no positive evidence as to their oi'igin. 



It is interesting to note that in the freshwater forms, S&ptaria 

 and Neritina, there are no spermatophores. Gilson has followed 

 •out the process of fertilization in X. fluviatUis^ and it is clear 

 from his account that spermatophores do not exist in this species. 

 Lenssen, it is true, alludes to the probability of their occurrence, 

 but he did not discover them, and my experience is the same 

 as his. I have found in one or two specimens a number of 

 spermatozoa agglutinated together in a mass of coagulum in the 

 vagina, but I could not detect any structure resembling that of 

 the spermatophores of Nerita and Paranerita. The I'educed size 

 of the spermatophore-sac in Septaria borhonica and ;S'. bougain- 

 villei afibrds evidence that spermatoiDhores are not formed in these 

 species. Against this must be set the fact that Thiele describes 

 a large spermatophore-sac in S. jo«rya, and in fig. 128 gives 

 the outline of a large iri*egular mass in its interior, but he 

 makes no mention of spermatophores. 



In fig. 57, «, b, c, d, e, I have given drawings of the different 

 ■forms of concretions found in the crystal sac of Nerita inelano- 

 traga. They dissolve readily in dilute acids with evolution of 

 bubbles, leaving an organic residue in which I could not find any 

 trace of a nucleus. They are composed of a number of crystalline 

 prisms radiating from the centre of the concretion and pi'ojecting 

 on the surface as shown in a and b. (Sometimes the crystals are 



