ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
455 
about in the pouch by means of the cilia of the large ciliated cells 
which form the head-vesicle. 
The foot is single and median, and though it shows no trace of a 
double origin, it may be considered as having arisen on both sides of 
the blastopore. At the posterior end of the embryo three or four large 
ciliated anal cells appear, and just ventral to them the distal end of the 
intestine is pressed against the ectoderm. The walls of the intestine are 
formed by small cells free from yolk. The supra-oesophageal ganglia 
appear as proliferations of the ectoderm on each side of, and dorsal to 
the mouth, and in connection with them, the eyes are formed as involutions 
of ectoderm ; the pedal ganglion is formed by delamination from the 
ectoderm at the sides of the foot. 
The segmentation of Urosalpinx is almost identical with that of the 
Oyster ; only the very earliest stages of this Mollusc were investigated 
owing to the great difficulty in cutting sections of the egg. 
Eyes of Pulmonata Basommatophora.* — M. Y. Willem points out 
that suitable sections show that the portion of the integument placed 
above the eye is almost entirely occupied by a vast lacuna. This is 
limited externally by a delicate wall formed of epidermis and of a layer 
of connective tissue in which there is neither pigment nor mucus- 
forming gland. The constant presence of blood-corpuscles and often of 
coagulated plasma in spaces which correspond to this cavity, shows that 
it is part of the general lacunar system of the body. Injections of the 
circulatory system of Limnsea stagnalis show that the pre-ocular sinus is 
the confluence of afferent and efferent canals distributed in part of the 
eye and especially in the tentacle. The author has observed the lacuna 
in the snail just mentioned, in L. palustris , Planorbis corneus , Physa 
fontinalis , and Aplexa hypnorum , and it is probably generally present in 
the Basommatophora. The morphology of this lacuna is easier to 
understand than its physiology. 
5. Lamellibranclaiata. 
Anodon and TJnio.f — Mr. 0. H. Latter has some notes on these 
animals. He first discusses the passage of the ova from the ovary to 
the external gill-plate, and thinks that this is effected by suction. With 
regard to the attachment of the Glochidia to the parent gill-plate it 
appears that the young attach themselves by their byssus, as the nutri- 
tive reserve in their neighbourhood becomes used up. It is very 
remarkable that the parent is able to draw back within the shell the 
long slimy masses of Glochidia even after they have been ejected a 
distance of two or three inches. It is not true, notwithstanding 
repeated statements to the contrary, that the young can swim ; they can 
be put easily into a state of great excitement by the introduction of the 
tail of a recently killed stickleback into the watch-glass in which they 
are lying. The Glochidium-shell has nearly always an influence on 
the shell of the adult, causing an irregular notch in the otherwise 
symmetrical curve. Schierholz is correct in stating that it is impossible 
to distinguish the sexes by their shells. All the fish with which the 
* Comptes Rendus, cxii. (1891) pp. 1378-80. 
t Proc. Zool. Soc. Lond., 1891, pp. 52-9 (1 pi.). 
