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3.—IlIHE DucriEess GLANDS. 
It will be most convenient to consider here a group of 
glands (the thyroid, thymus, spleen and suprarenal 
bodies), though these structures have a widely different 
morphological significance and have most probably very 
different functions. They agree in being glandular bodies 
devoid of efferent ducts, and acting in modifying the com- 
position of the blood either by adding to it some sub- 
stance (internal secretion), or by withdrawing some por- 
tion of its constituents. The lymphatic portion of the 
kidney is also supposed to function in some such way, but 
this structure will be most conveniently considered 
together with the renal organs. 
The Thyroid in P/ewronectes is not a compact gland, 
and is relatively very small in mass. It consists of a 
number of separate alveoli situated along the course of 
the ventral aorta. It is difficult to find by dissection in 
the full-grown specimen, and must be identified in a fish 
sufficiently small to section as a whole, or by microscopic 
examination of the tissues surrounding the vessel in 
question. The separate alveoli of which it is composed are 
not bound together in any way, but lie loosely in the 
connective and fatty tissue in which the ventral aorta is 
imbedded. They are most abundant in the immediate 
neighbourhood of the origin of the Ist and 2nd efferent 
branchial vessels, and lie mostly ventral to the aorta, but 
may be found lateral, and even dorsal, to it. In a trans- 
verse section through the region indicated in a small fish 
1} to 2 inches long there may be about a dozen alveoli 
present in a single section. Round the ventral aorta 
between the places of origin of the branches referred to 
and between the 3rd and 4th vessels few alveoli are pre- 
sent, though one or two may be found here and there. 
