150 



Crustacea. 



familiar adult form, it has an elongated body, with a six 

 jointed tail, the last somite bearing fairly well-developed side 

 appendages, this is the stage known to zoologists as the ' 'Mega- 

 lope " and which bears a general resemblance to the adult of a 

 form lower in development than itself, viz., the lobster. Prior 

 to this "megalope" stage it was of another form, known as a 

 Zoom, where it bore analogy to the adult of forms yet lower 

 down in the scale, — My sis, etc. Then, within the egg, it passed 

 through stages analogous to the early free swimming stages of 

 other, and still lower Crustaceans, viz., the "Nauplim " form. 



Now, taking the lobster, we have the same changes up to 

 the one in which we termed the great crab a "Megalope " but 

 which here in the lobster, becomes the final form ; and so forth 

 through all the class, even forms which to the general observer 

 would not be taken for Crustaceans at all — e g., the Ship Barn- 

 acle and that curious bag-like animal, devoid of power of 

 movement, form or comeliness, the Sacculina, the early stages 

 of these all bear witness to their common origin. 



The Ship Barnacle commencing life as do most Entomos- 

 tracans, and one or two of the prawns, (Penseus and Pasipheea) 

 viz., as a Nauplius, assumes later on the characters of another 

 Crustacean, or rather embodies the form of two in one. It 

 bears a bivalve shell like the Ostracoda, but within this, its 

 structure is approximate to that of the Copepoda (as shown on 

 diagram) finally it fastens to a piece of floating timber, and 

 assumes its familiar form the " Barnacle." 



In the Isopoda, the order to which belongs the common 

 " Woodlouse " of our gardens, the ancestral history is less easy 

 to read, but even here, in the maggot-like larvae, we can trace 

 the Nauplius outline, marred, but still recognizable, and cer- 

 tainly not more remote from the Nauplian form, than is the 

 parasitic mite of the human skin (Demodex follicularum) re- 

 moved from the Acarian type. 



I am not setting this forth as the precise and complete 

 science of the matter, but rather in a sense similar to that in 



