CRUSTACEA. 505' 



cavities among the vertebrata." The pigment is less a membrane than 

 an amorphous matter dififused through the outer layer of the superficial' 

 membrane, which changes to red in the greater number of species on 

 being immersed in alcohol, ether, acids, or water at 212'' Fahr. 



The calcareous crust of the animal is thick, and in the dorsal region 

 capable of great resistance ; their arms and legs are also of remarkable 

 hardness ; but in the smaller species the shell is often thin, and of 

 that crystalline transparency which permits of the functions of diges- 

 tion and circulation being observed. Many species, which are quite 

 microscopic, contribute colour to the sea — red, purple, or scarlet — 

 such are Grimothea Di/perreii, and G. gregaria. 



Before the year 1823 it was not generally supposed that this class 

 of animals was subject to change of shape in its larval condition, and 

 during its progressive development ; but about this time a certain able 

 microscopist clearly demonstrated that a minute nondescript kind of 

 animal, called the Zoea Tmiriis, was nothing more nor less than the 

 young of a kind of Prawn that had just escaped from the egg. This 

 able microscopist, Mr. Vaughan Thomson, of Cork, by many suc- 

 cessive observations, and under the fire of much adverse criticism, 

 satisfactorily established the truth of a metamorphic change taking 

 place in many genera, and, in particular, in the common crab {Cancer 

 nmnas); having succeeded in hatching the ova of this species, the 

 product of which proved to be a true Zoea. That there are variations 

 in this law of change has now been admitted, but that generally a meta- 

 morphosis exists analogous to that of insects in the various genera 

 of Crustacea with hardly an exception has been clearly established. ^ 



The recorded observations of the eminent naturalist we have men-' 

 tioned, Mr. Vaughan Thomson, as well as those of Mr. Couch, of 

 Penzance, Professor Milne-Edwards — and particularly those of the 

 last mentioned, who is the author of perhaps the best general work 

 extant on the Crustacea — are referred to as treating in detail on this 

 interesting subject. 



As an illustration of this metamorphosis, we give figures of the Zoea 

 Taurus in two states (Fig. 339), viz., a, in the first stage ; and second, 

 b^'2js, the animal appeared on the fourth day after the first micro- 

 scopical examination, and when it resolved itself into a kind of prawn. 

 These drawings appear in Mr. Bell's " History of British Stalk-eyed 

 Crustacea," and were taken by that gentleman from the work of a 

 Dutch naturalist named Slabber, who made the original observation 

 in: the year 1768, and published the result in 1778, from which time 

 the subject had been allowed to fall asleep until revived by Mr. 

 Vaughan Thomson. 



