CLASS OF CRUSTACEA. 485 



they acquire by the progress of age a greater number of limbs, 

 and there are some which completely change their form 

 during the early periods of life ; the lernsese offer us an 

 example of this transformation (Fig. 155). 



571. The class Crustacea, amongst which we must 

 arrange the cirrhipeda, placed by many naturalists, but erro- 

 neously, amongst the mollusca, is divided into five principal 

 groups, namely, 



The podophthalmaria, which have the eyes carried on 

 moveable peduncles, the anterior portion of the body fur- 

 nished with a carapace, ambulatory limbs, the mouth armed 

 with jaws disposed for mastication, and the organs of respi- 

 ration formed of branchiae properly so called. 



The edriophthalmia, whose eyes are not pedunculated, the 

 thorax exposed, the ambulatory limbs, the masticatory buccal 

 apparatus and the branchiae replaced by a portion of the 

 series of the limbs. 



The branchiopoda, in which the limbs are all foliaceous, 

 and perform at the same time the functions of fins and 

 branchiae. 



The entomostraca, in which the limbs are natatory but 

 not branchial, and in which the mouth is usually organized 

 for suction. Finally, the Xiphosuri, in whom the mouth has 

 no appendages which especially belong to it, but is sur- 

 rounded with limbs, the base of which performs the functions 

 of jaws. 



[When I first made the discovery that the vendace, the her- 

 ring, and many fine species of the salmonidae, live almost exclu- 

 sively on various kinds of the entomostraca, the view was 

 violently contested by naturalists ; and although the facts sub- 

 mitted to them admitted of no sort of doubt, to this day I have 

 not overcome their prejudices. The history of my inquiry was 

 afterwards taken up by my former student and assistant, Mr. 

 Henry Goodsir ; and his remarks, as those of an honest and 

 careful observer, may be found agreeable to the reader, more 

 especially as this most amiable and talented young man perished 

 in his youth. He was one of the companions of Sir John Franklin 

 in his last expedition. 



*' Hearing our fishermen often speak of 'something' which 

 abounded in great quantities in the Firth of Forth during the 

 summer months, which they called Maidre, and of which they 

 never could give me a clear description, I determined to examine 

 it for myself. 



"It was stated to me that this maidre was generally found 



