174 INVERTEBRATE PHYSIOLOGY 



similar studies among invertebrates. But application of these criteria has 

 not always been feasible with the invertebrates, so that conclusive proof 

 of endocrine function in certain physiological processes among these 

 groups has not always been possible. For example, one of the major ob- 

 stacles in the early studies dealing with endocrine regulation of chromato- 

 phores in crustaceans was the fact that the source of the chromatophore- 

 activating hormone was unknown, other than that it occurred in the eye- 

 stalk ; ablation of the eyestalks to remove the source of the hormone at the 

 same time removed the retina, which was necessary as a photoreceptor in 

 in the normal physiology of color change ; the limitation imposed on the 

 early investigators of this subject permitted at best the presentation of 

 strong presumptive evidence for endocrine participation in the regulation 

 of color change, through relatively gross deficiency and replacement experi- 

 ments. An additional criticism that could be directed against those early 

 studies which were Hmited to testing the efifects of injected tissue extracts 

 was that of specificity of the prepared extract, and the difficulty in dis- 

 tinguishing between pharmacological and physiological effects. In other 

 words, the extent to which tissue extracts were reproducing the normal 

 physiological process could not be readily determined. 



Hanstrom and his collaborators (Hanstrom, 1934, 1935, 1937; Carlson, 

 1935 ; Sjogren, 1934) soon placed crustacean endocrinology upon a more 

 substantial morphological basis by describing two apparently secretory 

 structures that occurred in the eyestalks of a number of species of higher 

 crustaceans. These structures were the X-organ and the sinus gland (first 

 called the "blood gland" by Hanstrom). Experimental attempts at localiz- 

 ing the source of the chromatophorotropic hormones led Hanstrom and 

 his colleagues to favor the view that the sinus gland was the origin, al- 

 though their experiments did not conclusively exclude the X-organ from 

 some role in color changes. 



Almost simultaneously in these same years appeared a number of 

 accounts which indicated that a number of physiological processes were 

 regulated or influenced by hormones originating in the eyestalk. In addition 

 to color change, which probably has been the most studied of the various 

 endocrine-influenced functions, those of retinal pigment migration, molt- 

 ing and growth, general metabolism, and some phases of reproductive 

 physiology have been the areas most closely investigated. But, despite the 

 impressive literature that has been built up in the past two decades, our 

 knowledge of invertebrate endocrinology is still fragmentary and incom- 

 plete compared to that of the vertebrates ; basic discoveries are still being 

 made ; basic questions are still unanswered. 



Instead of a detailed survey of the literature in this area, an attempt 

 will be made here to examine the present trends in crustacean endocrin- 



