786 On the Homologies of the Crustacean Limb. [October, 



trations — Messrs. Sinclair & Son having, at their own expense, 

 kindly struck off an edition of the accompanying plates from the 

 drawings on stone made by them for the Survey. 



The reader is supposed to have a general knowledge of Crus- 

 tacea, especially the Phyllopods, a brief account of which may 

 be found in the author's Zoology, where the genera here referred 

 to are figured. As to the anatomy of these interesting Crusta- 



FlG. l.— Limn .'/, /; :■//>. ,.■■. enlarged. Burgess, del. ' 



cea, a transverse section of the anterior part of the body of any 

 genus of Phyllopods (see PI. xn, Fig. 2, also Fig. 1 in text) will 

 convey an excellent idea of the leading features in their organiza- 

 tion, especially those by which they differ from the members of 

 other Crustacean orders. The leading topographical features in 

 the body, particularly of Arthropods, are the form of the elemen- 

 tal segments with their appendages, and the relations of the prin- 

 cipal anatomical systems to the body-walls. 



Genera/ relations of the systems of organs to the body ic alls. ■ 

 We will first look at a section of a typical Phyllopod, such as 

 Apus (Fig. 2). The body-walls are rather thick and the muscles 

 are well developed, particularly the dorsal extensor muscles, and 

 the motor or extensor muscles of the limbs, which arise in part 

 from the dorsal region, and in part from the sides and sternal re- 

 gion. The body cavity is rather small. The heart is large, either 

 cylindrical as in Estheria, or flattened as in Thamnocephalus. The 

 digestive tract is large, capacious, and the cavity of the head is 

 mainly filled with the two liver masses ; the brain being remarkably 

 small, while the nervous cord, especially the second and succeed- 

 ing ganglia, are remarkably small and weak, compared with other 



