872 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



After describing the development of the spermatozoa, which is 

 done in very great detail, and illustrated by numerous drawings, the 

 author proceeds to sum up the known cases in which a similar 

 dimorphism of the spermatozoa exists ; these are Notommata Sieboldii, 

 Asellus aquaticus, Oniscus, Cypris. The same phenomenon has been 

 recently observed in a species of Murex by Schenk, The author 

 himself records it in Ampullaria, and gives some description of the 

 structure of the two kinds of spermatozoa, but was unable to find any 

 but the " hair-like " form in other Prosobranchiata. 



3. Function.- — The question to be resolved is, What share does 

 each kind of spermatozoon take in fertilization ? Leydig previously 

 stated that both kinds were concerned in the fertilization of the 

 ovum, but a careful series of observations has resulted in the con- 

 clusion that only the hair-like spermatozoa penetrate the ovum ; 

 the worm-Uhe spermatozoa play no part in fertilization, and their 

 actual function is not easy to settle. It is, however, well known 

 that in the testis of many animals, comprising examples from various 

 groups, a great number of cells do not become spermatozoa, but in- 

 crease in size and take on the appearance of ova, so that there is a 

 kind of commencing hermaphroditism. It is possible that thei 

 " worm-like " spermatozoa of Paludina may be something analogous, 

 produced by " a certain female tendency in the testis." Eemembering 

 also that the nearest allies of the Prosobranchiata are hermaphrodite, 

 there seems nothing too improbable in imagining that they too may 

 show an indication of hermaphroditism in their genital glands. 

 Finally, a comparison of the structure of the testis of Paludina with 

 the hermaphrodite gland of the Pulmonata shows a very considerable 

 correspondence between the seminal cells which are to produce the 

 worm-like spermatozoa in the one, and the ova in the other. In the 

 hermaphrodite glands of the Pulmonata the spermatozoa and ova are 

 developed in alternating masses ; and the same is the case with the 

 two kinds of spermatozoa in Paludina. All these considerations 

 seem to show that the testis of Paludina and the hermaphrodite gland 

 of the Pulmonata are very closely connected. 



The author concludes this very important paper by a suggestion 

 supported by many facts, that the Pulmonata are more nearly allied 

 to the Prosobranchiata than to the Opisthobranchiata, as is more 

 generally supposed. 



Mode of Action of Shell- and Rock-boring Molluscs.* — Prof. 

 F. H. Stoner considers the true explanation of the mode of action 

 of many shell- and rock-boring molluscs to be that there is a power 

 of osmotic dissociation, similar to that possessed by the rootlets of 

 plants, and that it depends on the formation of chlorhydric acid 

 through decomposition of sea-salt by certain tissues of the animals 

 especially suited for the purpose, these tissues being kept meanwhile 

 in direct contact with the shell or rock to be bored. In the case of 

 shell-perforation the denticles of the odontophore would aid by 



* Amer. Journ. Sci., xxviii. (1884) pp. 58-61. 



