CRUSTACEA. 



cephalic, or any abrupt line of demarcation; and the fact that the 

 mEgans of one species, even to the mandibles, may in others be 

 devlped into feet, and conversely the feet may become mouth-c,gans 



But there are other evidences of equal importance. It is a fact of 

 much weight that the obsolescence of members takes place commonly 

 at the extremities of the cephalothorax, and at the extremities of the 

 abdomen In the former, the pedunculate eyes and anterior antennae 

 may become obsolete at one extremity, and one, two, or three pos- 

 terior pairs of thoracic legs at the other, the exterior pair m each case 

 being the first to disappear. In the abdomen, the basal segment and 

 the apical are often obsolete in the lower Crustacea. A table given 

 on a following page illustrates this point. 



The pedunculate eyes are absent from a large part of Crustacea, 

 and in some Daphnida3, the first antennae also are obsolete. In 

 some Schizopoda, the last thoracic feet are wanting, in others, the 

 last two pairs; in Cyclops, the last two or three pairs ; in Daphnia. 

 the last four; in Cypris, the last six pairs; while the intermediate 

 organs in each of these cases are all present. 



It hence appears that the cephalothorax and abdomen should each 

 be viewed as a whole, in which the extremities of each, according to a 

 fundamental law, fail of developing the full allowance of members. In 

 the Caprellidse there is a seeming exception, since here the feet near 

 the middle of the cephalothorax are often obsolete. But these cases 

 do not set aside our conclusion; for the feet which fail are not the an- 

 terior thoracic feet, and therefore they do not mark or indicate any 

 subdivision between a head and thorax. A general survey of the 

 facts seems to show, that the cephalothorax and abdomen are each a 

 distinct centre of development, in which progress reaches to a wider 

 or narrower circumference in different species. 



Embryology sustains us in this deduction. The abdomen in the 

 growing germ appears as a mere point, almost as soon as a trace of 

 the anterior part of the body appears and before any members can be 

 distinguished, and it is a separate centre of development. In the head 

 and thorax united there is but one other centre, and from it progress 

 goes on either way anteriorly and posteriorly. The anterior part of 

 the buccal mass marks this centre; the mandibles are the first organs 

 that begin to appear, and, at the same time, rudiments of the upper lip 

 may be traced; then the posterior and anterior antenna* commence 

 and the former (or the organs next before the mandibles), are mosl 



