rackard.] 286 



The scutelluin Is now in contact with the base of the abdomen, as if 

 the whole thorax had been carried backward, and the entire abdo- 

 men brought for^vards and upwards, due to the absorption of the meta- 

 thoracic ring and basal ring of the abdomen. 



Thus each of the three regions of the body is a centre of develop- 

 ment, the gradual perfection of the appendages belonging to each 

 region proceeding from the centre towards the periphery ; beginning 

 at the insertion of the limbs to the trunk, and gradually perfecting their 

 development towards the extremity. Hence the wings, the tarsi, or 

 terminal joints of the limbs, and the abdominal appendages, are the 

 last to be developed and perfected. The anterior part of the thorax 

 is perfected earlier than the posterior ; while in the abdomen, the 

 development goes on from behind forwards. Prof. Dana has sliown that 

 in the Crustacea the cephalothorax and abdomen are each a distinct 

 centre of development, in which progress reaches to a wider or nar- 

 rower circumference in ditferent species.* Researches on the embry- 

 ology of the higher Annelids show that the development of worms 

 proceeds from a single centre.f 



At this stage, which may be properly called the pupa state, the 

 eyes begin to turn dark, and a few hairs develop themselves upon the 

 upper side of the abdomen ; but the stage is so transitory that in a 

 long series of Individuals it is impossible to select a single individual, 

 and denominate it a pupa, since there is no pause in the metamor- 

 phosis for a special biological design, such as obtains in the Lepidop- 

 tera and majority of lower Insects. The terms larva, pupa, and 

 imago, are not therefore absolute terms. 



SUBIMAGO STATJE. 



Certain Individuals which would upon a casual glance be mis- 

 taken for "pupffi," differed so much from what we have called pupae 

 above, that they may be said to be analogous to the suhimago state of 

 Ephemeridte. In this state the arthroderm, owing to the rapid deposi- 

 tion ofchitine, is more dense and harder ; the wings are as large as in 

 the perfect bee, and the joints of the legs are spiny, while the ovij)os- 

 itor has become wholly withdrawn within the walls of the abdomen. 



In some specimens, remains of a thin pellicle were found upon the 

 extremities; so that we are neither justified in calling this Individual 

 an imago, or on the other hand, a pupa. The individuals had not 

 left their cells. Their feet had not yet been used for purposes of 



* Introduction to the Crustacea of the U. S. Exploring Expedition. Vol. 1. p. 22. 



t See S. Loven. K. Vetenskaps, Acad. Handl. 1840. VViegraann's Archiv, 1842. 

 Parti. M. Sars. Development of Polynoe cirrata. Wiegmann's Archiv, 1845. 1'ai't 

 I, Milne Edwards. Anuales Science Nat. 1845. 



