594 ORGANS OF SECRETION. 



like the vibratile cilia of many inferior tribes. The cellular 

 follicles covering the great venous trunks on the ventral 

 surface of the liver, and communicating freely with their 

 interior, have been compared by some to urinary organs, 

 and by others to the rudiments of a portal circulation. The 

 genital glands here manifest the same elevated character of 

 development as most of the other secreting organs ; the tes- 

 ticle forms a large conglomerate mass composed of long, 

 small, divergent, ramified tubuli seminiferi, with their closed 

 extremities forming the periphery of the organ. The long 

 vas deferens is convoluted like an epididymis, and is ac- 

 companied with a vesicula seminalis and a prostate gland. 

 The ovary forms a large cluster of highly vascular secreting 

 follicles, and the oviducts are furnished with distinct glands 

 to secrete the enveloping materials of the ova, as in the 

 plagiostome cartilaginous fishes. 



The higher development of the secreting organs of the 

 molluscous classes, is thus indicated by the greater number 

 and greater sub-division of their component tubuli, and 

 the more extended surface thence presented for the dis- 

 tribution of the blood-vessels which supply the materials 

 of the secretions. It is seen in the concentration of all the 

 constituent follicles and tubuli into single efferent ducts, 

 and in the elongation of these ducts, by which the secreting 

 glands are removed to a greater and more convenient dis- 

 tance from the parts whence they originate and which 

 receive their secretions. The increased extent likewise of 

 the vascular system in the molluscous classes, and the minute 

 distribution of arterial capillaries through the glandular 

 tissues, augment the products of the secerning organs. The 

 efferent ducts are more distinct in form, structure, and func- 

 tion, from the follicular secerning portion of the glands; they 

 terminate with more oblique and valviform orifices, and the 

 analogies of form, connections, and function more closely 

 approximate the glands of mollusca to the vertebrated type, 

 than those of the articulated or radiated animals. 



FIFTH SECTION. 



Secreting Organs of the Spini-cerebrated or Vertebrated 

 Classes. 



As the general development of the glandular system of 



