FROM EMBRYOLOGICAL DATA. ' 25 



there is a distinct inferiority of development in all these respects among Crustacea 

 when contrasted with true Insects, which should leave scarcely a doubt in our minds 

 that this class as a whole is inferior to the class of Insects as a whole. 



But there is another point in this question Avhich, in my opinion, settles definitely 

 the relative position of Crustacea between "Worms and Insects. Wherever the joints 

 of their body unite to form distinct regions, they combine in such a manner as to 

 form only two well-defined divisions : the tail, or rather abdomen, with free, movable 

 joints, and the rings of the head and chest, united into one continuous shield above, 

 though below they may present more or less distinct indications of their primitive 

 separation. Head and chest united, and abdomen free and movable, with morpho- 

 logically homogeneous appendages upon all the joints, — such is the character of 

 the highest Crustacea. Now, in the development of true Insects we have a stage of 

 growth when these animals assume precisely the same condition. It is that of the 

 pupa. Before the insect assumes its final winged state, the rings of the anterior part 

 of the body are soldered together more closely, and the abdomen alone remains free 

 and movable. The lower wings, with a somewhat gill-like structure, which, for a 

 time, are free in the young pupa, are soldered upon the inner surface of the upper 

 wings, Avhich themselves adhere to the chest. The jaws and legs are almost iden- 

 tical, and it may be said that a pupa corresponds, in its main features, to that com- 

 bination of characters among Articulata which is peculiar to the Crustacea. Indeed, 

 what is the pupa among Insects ] It is that state in their development in which the 

 anterior joints, which were free in the larva, are fused into one distinct region, no 

 longer endowed with free movements ; but in which the joints of the abdomen 

 still remain movable. This answers most remarkably to the structure, form, and 

 adaptations of the body of Crustacea. And this being the case, I consider the 

 position of the whole class as settled. It is intermediate between Worms and In- 

 sects ; for, in the general development of Insects, the pupa state, which corresponds 

 to the Crustacea, is the intermediate state between the worm-like larva and the per- 

 fect insect. Worms, therefore, are the lowest class among Articulata, the Crustacea 

 the next class above, and Insects the highest class of that type. Of course, in this 

 arrangement we give up all possibility of bringing the higher type of Articulata in 

 any way near the lower types of any of the other great divisions of the animal 

 kingdom, with which it might seem natural to combine them in one continuous 

 series ; as, for instance, with the lowest of the Vertebrata. But such attempts are 

 precisely what I have objected to in our classifications ; while, upon the arrangement 

 I propose, the type of Articulata in all its peculiarities is gradually wrought out 

 more and more fully, and in its highest class recedes most from all other types. 

 The impossibility of combining the type of Articulata, by its highest families, with 

 any of the other types, I consider as one of the most valuable features in my classi- 

 fication. I know very well that some of the Annellides have been mentioned as 

 constituting a link between Articulata and Vertebrata through the Myxinoids. But 

 who would, in the present state of our knowledge, place Annellides above In- 

 sects and Crustacea, because they have colored blood, when their relation to Hel- 

 minths is so plainly ascertained 1 It seems to me far more rational to trace the 



VOL. II. ART. 6. — 4 



