ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
669 
region, the flagellum, and other parts are described briefly, but a full 
account is promised. In the so-called receptaculum seminis or copu- 
latory vesicle, no active spermatozoa were found. 
Arrangement, Correlation, and Function of Pallial Organs of 
Tectibranchiata.* — The general results of Mr. J. D. F. Gilchrist may 
be thus summed up ; while the presence of a shell is characteristic of 
one large division of the Mollusca, its absence is just as characteristic 
for another. The result of the calcareous covering is special adaptation 
for respiration, in the form of a ciliated gill in the mantle-chamber, 
which must, for respiratory purposes, produce a sufficiently strong stream 
of water. This stream leads to the development of a special sensory 
organ (osphradium) at a point where there is a suitable nerve-supply. 
In connection with this there may appear an external organ, the siphon, 
which is well developed in many Prosobranchiata ; in general it has a 
tendency to approximate to the cephalic or sensory region. In the 
Tectibranchiata it is wanting, and its absence may be explained by 
the distance the mantle-cavity is from the cephalic region. 
When the animal becomes inedible there is a new turn ; it can come 
out from its shell ; large parapodia, too large to be withdrawn into the 
shell, are developed, and serve for swimming or crawling ; these para- 
podia may become powerful respiratory organs; the gills and the 
water-stream become less necessary, and may disappear ; the head-region 
is the most suitable place for sensory stimuli from the surrounding 
medium, and this may lead to the development of rhinophors, and the 
gradual disappearance of the old osphradium. And with this there may 
be the loss of the incurrent siphon and the backward movement of the 
pallial cavity. The filling up of this last and the disappearance of 
the shell are correlative changes. All these changes may be very well 
seen in existing Tectibranchiata. 
But the study of the “ pallial complex ” shows not only the further 
development of the Prosobranchiata, but also an approximation to the 
Nudibranchiata. The respiratory processes are shown by Pleurobranchsea 
and Umbrella to be either parapodial, or processes of the mantle or of 
the general body- wall. 
That the osphradium is developed in direct relation to the water- 
stream, and in indirect relation to the rhinophors, is made still clearer 
if we compare it with similar phenomena in air-breathing animals. The 
Prosobranchiata have a specialized external organ in the stream ; the 
Opisthobranchiata, like Insects, in which there is no special localized 
stream, have an olfactory organ in the cephalic region. 
With regard to differences in the structure of the gills, it is pointed 
out that the gill of the Aplysia- type, which is characteristic of the 
Cephalaspideae and Anaspidesc, stands out from the body,*and forms 
almost a right angle with its long axis ; those of the Notaspidese, in 
which the gills are only incompletely protected by the mantle, lie close 
to the body, and have their axis almost parallel to it. 
Reversal of Cleavage in a Sinistral Gastropod.f — Mr. H. E. 
Crampton has a brief account of some observations on the early stages 
* Jenaische Zeitschr., xxviii. (1894) pp. 408-59 (21 figs.), 
f Ann. New York Acad. Sci., viii. (1894) pp. 167-70 (1 pi.). 
