ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 81 



Amphipoda, that we are enabled to determine the true homologlcal relation 

 of one part to the other. 



In all Crustacea above the Entomostracous forms the several somites are 

 distinguished by a dorsal and a ventral arc. The dorsal is invariably a hard, 

 strong, and osseous plate. The ventral arc is mostly represented by an osseous 

 band that reaches across the animal, aud is united anteriorly and posteriorly 

 to the contiguous somites by large and flexible membranous tissues. The 

 dorsal arc is wide, and dips under the adjoining one anteriorly in all except 

 the second somite in the Macrura, which overrides the plates of the adjoin- • 

 ing somites both anteriorly and posteriorly. This arrangement does not 

 exist in the Edriophthalmia, because, there being no dorsal carapace pro- 

 tecting the pereion, all the somites have a separate and distinct dorsal arc. 

 The consequence is that each somite posteriori}'- overlaps the anterior margin 

 of the next succeeding ring, except the first or anterior somite of the pereion, 

 which overlaps anteriorly the posterior margin of the cephalon and posteriorly 

 the anterior margin of the second somite of the pereion. In each of these 

 orders of Crustacea we find that the greatest power of flexion is given 

 to the animal at these points. 



In all the distinguishable somites of the Edriophthalmia, from one extre- 

 mity of the auimal to the other, each separate one is observed to support 

 laterally a large plate. These, in the pereion, are firmly attached to their 

 respective somites, but not ossified to them ; in the pleon they are so united 

 by ossific matter that one part is not capable of being separated anatomically 

 or distinguished in structure from the other. It is these parts in this 

 particular division of Crustacea that originated the idea of the theory of the 

 Crustacean somite as enunciated in 1830 by Prof. Milne-Edwards. The 

 fact that the supposed side-plates, or epimera, were merely the first joint of 

 the normal legs or appendages has been satisfactorily demonstrated in the 

 Edriophthalmia, as far as relates to the somites of the pereion ; but hitherto 

 the I'elation of the side-plates of the pleon to the normal condition of the 

 mobile appendages had not been demonstrated until the structure of the 

 -dermal anatomy of the genus A23seHdes had been made out* : that " one inter- 

 esting and, as far as we know, unique feature in these Crustacea yet remains 

 to be noticed. The segments of the pleon have the lateral walls (long known 

 as the epimera of Milne-Edwards, called also the pleura by many authors) 

 existing as articulated appendages, demonstrating two important features in 

 the homologies of these parts: 1st, that they are all really portions of the 

 appendages, being the first joint or coxae of the pleopod . . . aud 2ud, that, 

 since the peduncle consists of three joints, the second branch in the appen- 

 dages of the pleon, as in other parts, is shown to take place invariably at the 

 extremity of the third joint." In the Macrura and higher Stomapods the 

 coxal joint of the several appendages is united to the dorsal arc in a ver}' 

 perfect and complete state of ossification, with the exception of the first 

 somite, where there are no appendages, and the sixth, where the coxa is free 

 and articulates, with small lateral motions, with the dorsal arc of the respec- 

 tive somite. The seventh somite {teIso7i) is reduced in character and altered in 

 foi-m ; it universally covers and holds the terminal exit of the alimentary 

 canal, the inferior arc of which is represented by a membranous tissue. In 

 the Amphipodous order of Crustacea the fifth and sixth somites carry their 

 appendages with free coxa;, and the terminal somite exists only in the form 

 of a scale very liable to vary in shape, or separated into two of minute 



* Hist. Brit. Seasile-eyecl Crust, vol. ii. p. 146 {Aimeudcs), 

 1876. G 



