ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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nerve, which takes an undulating course between the organs and the 
bases of the legs. 
There is a precocious separation of ectoderm and endoderm (yolk- 
cells) during the formation of the blastoderm. The endoderm retains 
its primitive character as a solid mass of long yolk-cells until the 
caudal spine appears. The yolk-cells are not true vitellophags ; they 
metabolize the yolk which is contained in each, but the cells themselves 
arc directly converted into the living epithelium of the mid-gut. 
In embryos at the time of hatching the sternal artery has arrived at 
the condition found in the adult scorpion ; it consists of a tube which 
lies on the upper surface of each half of the oesophageal nerve-ring. 
It is not till much later that it obtains the investing character of the 
adult. Packard’s brick-red gland is of mesodermal origin ; it contains in 
its interior the cavity of the fifth postoral somite ; it soon becomes 
folded on itself and the region of the bend grows rapidly forwards. The 
outer limb of the fold becomes folded at four points and these new bends 
grow out in each body-segment, giving rise to the lobes which are 
characteristic of the organ in the adult. With the folding there is a good 
deal of fusion of the walls, and this is followed by perforations, the 
result of all of which is the peculiar anastomosing structure of the 
adult organ. The author thinks he has afforded further evidence of 
the close relationship between Arachnids and Limvlus and reasons for 
removing the Merostomata more widely from the Crustacea. 
6. Crustacea. 
Excretory Apparatus of Decapod Crustacea.* — M. P. Marchal, in 
continuation | of his studies on the excretory apparatus of Decapod 
Crustacea, describes that of Homarus vulgaris. The antennary gland is 
large and heart-shaped ; the saccule has a cavitary system composed of 
elegant ramifications which radiate around its orifice. The orifice itself 
is bordered by clear cells which are very high, and it leads into the 
second portion of the gland which may be called the labyrinth ; this is 
large and is divided into two lobes forming a U, the outer branch of 
which communicates with the saccule. The labyrinth is formed of a 
number of extremely fine canaliculi which anastomose with one another 
in all directions and so form a close spongy tissue, the innumerable 
lacunae of which are invested by an epithelium of striated cells, covered 
by a cuticle. 
In Palaemon serratus the saccule is small, reniform, and independent 
of the rest of the gland, with which it is connected only at its point of 
communication ; its cavitary system is formed of a central space and of 
short areolar diverticula which are given off from it. The orifice of 
communication is wide, and bordered with very high granular cells. 
The labyrinth forms a spongy, rounded mass. The two bladders have 
numerous prolongations which ramify among various organs. In front 
of the stomach they unite to form an unpaired suprastomachal bladder 
which has the form of an elongated sac with smooth walls. 
In Pagurus Bernliardus the saccule is ramified, and there is the same 
difference between it and that of the two preceding types as there is 
* Comptes Rendus, cxi. (1890) pp. 458 60. f See ante, p. 324. 
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