CRUSTACEA. 



147 



embryonic forms. One of these stages is known as the Zoea, a name used at 

 first, just as was the case with nauplius, under the belief that the animals 

 included were adult. For the discovery of their true relations we are indebted 

 to Mr. Thompson, who also was the first to discover the metamorphosis of 

 the barnacles. Previous to his time two or three had studied the growth 

 of the cray-fish, which passes directly from the young to the adult condi- 

 tion by gradual stages and without any abrupt transitions; and hence 

 when Mr. Thompson, in 1828, published a memoir, entitled "On the Meta- 

 morphosis of the Crustacea, and on Zoea, exposing their simple structure, 

 and demonstrating that they are not, as has been sup- 

 posed, a peculiar Genus, but the Larva of Crustacea ! ! ' 

 the amount of discussion provoked was considerable. 

 Every naturalist at once proceeded to controvert him, 

 citing this or that fact and this or that authority as 

 being totally opposed to him and his views. But at last 

 the correctness of his observations were acknowledged, 

 and the Crustacea were recognized, not, as had been 

 stated in the Encyclopaedia Britannica, as undergoing 

 no metamorphosis ; but, on the other hand, as being the 

 group where metamorphosis was most nearly universal. 

 Our figure shows the general type of Zoea : the long 

 dorsal spine is characteristic of most of the crabs. 



We have already figured one of the porcelain crabs 

 and its young on a preceding page (Figs. 112 and 113), and we only 

 need to refer to them here as being- among the most beautiful and cleanly 

 of all the decapod hosts. As both their common and their scientific 

 names [Porcellana) in- 

 dicate, their shell has 

 the appearance of por- 

 celain, while its bright 

 colors and the scrupu- 

 lous care with which 

 they keep it clean adds 

 to its beauty. In con- 

 trast to these forms 

 are some other tropi- 

 cal species known as 



Dromia Thev seem to FlG - 130 - _ A Chinese crab (Dromia), with a sea-anemone on its bark 

 ■J "" See page 30. 



delight in keeping their 



bodies covered and concealed by all sorts of foreign objects. Sometimes 



the back will be covered by a waving grove of hyclroids or by the irregular 



Fig. 129. — Zoea stage of 

 common European crab. 



