576 ORGANS OF SECRETION. 



which appear to attain no farther development than that of 

 a simple deciduous blastema. 



As a follicle implies merely a more elevated condition of a 

 secreting surface, than a simple flat membrane, an aggregate 

 follicular gland, merely a more advanced condition of a 

 follicle, and a conglomerate glandular mass, merely a further 

 development of a follicular gland ; and as the same gland 

 passes through all these forms, in the course of its develop- 

 ment : so we find the highest of these conditions, the con- 

 glomerate form, to characterise the glands of the highest 

 animals ; and, in the lowest radiata, the cutaneous and the 

 mucous surfaces afford all the requisite secretions, without 

 even assuming the follicular form. All secreting glands, so 

 far as their structure can be perceived, being thus merely 

 membranous coecal tubes, with highly vascular parietes, their 

 peculiarities relate chiefly to trivial differences of form, size, 

 situation, and products, throughout the animal kingdom, 

 which admit of being considered either in zoological order, 

 or by following separately, through all classes, the history of 

 every individual gland. 



SECOND SECTION. 



Secreting Organs of the Cycloneurose or Radiated classes. 



In the radiated classes of animals, which constitute the 

 first stages of animal organization, the glandular form ap- 

 pears rarely necessary for affording the various secretions of 

 their cutaneous or mucous surfaces ; a copious mucous 

 secretion is poured out from every part of their soft smooth 

 cutaneous exterior, and from the mucous lining of their 

 digestive cavities, without their appearing to require distinct 

 cryptee, or other forms of glands for its formation. The 

 earliest developed, and the most constant gland in the animal 

 kingdom, is the liver, the first formed and the most compli- 

 cated in the highest animals, and the most important in the 

 process of nutrition ; and in the radiated, as well as in most 

 of the articulated animals, this organ, so complicated and 

 enormous in the vertebrata, generally consists of simple 

 isolated follicles, more or less numerous, opening directly 

 into the cavity of the stomach. The numerous intestinal 

 coeca, or stomachs of poly gastric animalcules, opening into 



