ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
23 
to the cerebral or to the pedal — a matter of importance, as in the lower 
Prosobranchiata the pallial ganglia are in close connection with the 
pedal, and as the series ascends they enter into closer relations with the 
cerebral. In Paludina they lie from the first midway between the cerebral 
and pedal ganglia. The intestinal ganglia arc in the adult very feebly 
developed ; but it is to be noted that Paludina has some very primitive 
characters, in so far as the ganglia are very diffuse, and it is consequently 
very hard to say where a ganglion begins and where it ends ; indeed 
the commissures and connectives are invested for some distance by a 
layer of ganglion-cells. The long pedal cords are throughout ganglionic 
in nature. 
The sensory organs are next discussed ; an otolith-vesicle is formed 
on either side of the foot a little before the eye, which appears at the 
base of the tentacle ; the lens and vitreous body are certainly formed by 
cells of the optic vesicle. Special attention is given to Spengel’s olfactory 
organ, the development of which can be very clearly followed in Paludina. 
It consists of an elongated ridge, rounded in cross-section, traversed for 
the whole of its length by a nerve which arises from the supra-intestinal 
ganglion ; there are on it a series of pit-like depressions of the epithelium 
of the mantle-cavity, about twenty in number, set along the inner edge 
of the ridge, and partly covered by it. Spengel’s organ arises as an 
ectodermal swelling, and is at first set obliquely to the longitudinal 
axis ; during the course of development it alters its position until it 
becomes parallel to the long axis. The pits appear gradually, and are 
invaginations of the mantle-cavity. A nerve from the supra-intestinal 
ganglion soon passes into the ridge. 
The vessels arise quite independently of the heart, either between 
the splanchnic mesoblast and the endoderm or the parietal mesoblast 
and the ectoderm. They soon become surrounded by a thin layer of 
connective tissue, and extend into the mesoderm as lacunar spaces. The 
first foundations of the vessels appear before there is any indication of a 
heart. A contractile sinus, which corresponds to a kind of embryonic 
heart, is formed, and, later on, becomes part of the anterior aorta ; a sinus 
which is formed later on and lies ventrally to the liver, passes into the 
hinder aorta. Venous sinuses are, at a very late period, formed in a 
remarkable way, for the mesoderm around the intestine, which is at first 
compact, withdraws from it and so forms a lacunar space. 
The gonads have the same kind of foundation in both sexes ; the 
germ-gland and duct arise separately from one another. The former 
appears as an outgrowth of the pericardium, or, in other words, of what 
remains of the secondary coelom that is not filled up by connective tissue 
and muscles. This outgrowth becomes constricted off and sooner or 
later enters into connection with the primary genital duct ; this duct 
is in itself an outgrowth from the ventral side of the ureter. Both 
rudiments are well seen when the larva has a distinct velum and a very 
well developed pair of primitive kidneys. 
In the male a small part only of the seminal duct is formed from 
the primary genital duct— that part, namely, which extends from the 
testes to the angle of the vas deferens. The remainder has a secondary 
origin, independent of the primary duct, for it ajjpears as a groove which 
extends from the floor of the mantlo-cavity to the region of the head. 
