40 Elementary Zoology. 



the embryo to that whence the other appendages arise, it is 

 probably not justifiable to make this comparison. 



We thus see that the crayfish body consists of nineteen 

 segments, each provided with a pair of appendages, a rostrum 

 in front and a telson behind. That the segments are less clear 

 anteriorly, as is the case with the earthworm, where their 

 characters are less marked {i.e. absence of setse on the first, 

 absence of distinct septa internally, etc.) ; there is, in fact, by 

 this means, a specialized anterior region — a head formed. The 

 limbs are reducible to a common plan, which is modified in 

 accordance with the varying function of the.limbs. 



In the course of the dissection necessary to follow the fore- 

 going description, the organs of respiration — the^///j' or branchia 

 — will have been noted. They are feather-like structures lymg 

 beneath the branchiostegite, and limited in consequence to the 

 thoracic region of the body. On each side there are altogether 

 eighteen fully developed gills, besides certain rudiments which 

 will be referred to in due course. These gills, however, are 

 arranged distinctly in three series, and according to their 

 position have been termed podobranchs, arthrobranchs, and 

 pletirobranchs . When the thoracic limbs (7-12 inclusive) are 

 removed a gill will be removed also. If care is taken, only 

 one gill will be torn away with each limb. These gills, which 

 are attached to the coxopodites of the limbs, are the podo- 

 branchs. Each consists of a stem which bears a series of 

 filaments on each side. The stem expands above into a 

 lamina which is bent in the form of an open book. On the 

 first maxillipede there is no podobranch; at least, no fully 

 developed podobranch. There is, however, the structure 

 already described as the epipodite. It is nearly certain that 

 this membranous plate is a rudimentary podobranch; and 

 these are the reasons which lead to that inference. In the first 

 place it occupies precisely the same relations to the limb 

 that bears it as do the podobranchs of the six following gills. 

 Secondly, it is a longitudinally folded plate, like the lamina of 

 the gills, and bears certain hooked spinelets upon its surface 

 exactly comparable to spinelets borne upon the lamina of the 

 fully developed gills. But the strongest argument is that in 



