SEA-FISHERIES LABORATORY. 217 



3. — The Ductless 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 Pleuronectes 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 1st 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. 



