6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



ment ; in those having antennal glands they he in the second maxillary 

 segment. Each gland is implanted on the epidermis by an enlarged 

 base and is innervated from the suboesophageal ganglion ; in form it is 

 conical, lenticular, or f oliaceous. From their histological structure and 

 changes during the intermoult period, Gabe shows that the Y organs 

 are comparable to the thoracic endocrine glands of holometabolous 

 insects, and he suggests that they have something to do with moulting. 

 Following this suggestion, Echalier (1954) made experimental tests 

 by removing the organs. He found that bilateral ablation of the 

 glands, when not made too late after they had already discharged their 

 secretion, resulted in a very great lengthening of the intermoult period, 

 far in excess of the usual time between moults. Echalier, therefore, 

 contends that the Y organs are crustacean endocrine glands for the 

 control of moulting. That they do not disappear in the adult as do 

 the thoracic glands of insects, Gabe points out, follows from the fact 

 that the crustaceans continue to moult in the adult stage. 



I. EVOLUTION OF THE ARTHROPODS 



In any discussion of arthropod metamorphosis the question of re- 

 capitulation always comes up in relation to the larval forms. If there 

 is any ancestral recapitulation in ontogeny, it then becomes necessary 

 to have at least a theoretical concept of the evolution of the arthropods 

 and some idea of what ancestral forms they had that might be re- 

 capitulated in the development of the individual. 



The evolutionary origin of the arthropods is hidden in remote Pre- 

 Cambrian times, so probably we shall never know the facts from visual 

 evidence. There is, however, ample evidence from a study of modern 

 forms to indicate that the early progenitors of the arthropods were 

 closely related to the progenitors of the annelid worms, and that these 

 two groups of annulate animals had a common ancestor. The funda- 

 mental characters preserved in the annelid-arthropod organization 

 are : an elongate segmented body, an alimentary canal extending 

 through the length of the body, a paired ventral nerve cord with seg- 

 mental ganglia, a somatic musculature, and mesodermal coelomic sacs. 

 We may therefore visualize the primitive annulate as a very simple, 

 wormlike creature having these features. The mode of development 

 was anamorphic, new segments being formed in a subterminal zone 

 of growth. From this primitive segmented worm the annelids have 

 been directly evolved with little addition other than the development 

 of segmental groups of lateral bristles, which in the polychaetes have 

 been carried out on movable lateral lobes of the segments, the so-called 

 parapodia, that serve for swimming and burrowing. 



