REPORT ON THE CRUSTACEA MACRTJRA. 



Xlll 



The Ophthabnopoda. — Of the several somites that compose the body of the Decapod 

 Crustacea, that which supports the organs of vision is the most anterior. This can be 

 demonstrated by the course of the progressive development, even of the forms which 

 depart most from a simple type, as well as by means of dissection, the most anterior 

 branches given off from the cephalic ganglion going directly to the organs of vision. 

 Theoretically, these organs are the lateral appendages of a somite which in many 

 genera is not traceable ; but among the Macrura it is frequently present in the form of 

 a more or less distinct calcified bar, lodged between the inferior surface of the projecting 

 front of the carapace and the tergal portion of the second or antennal somite (PI. CXIII. 

 fig. la-a), which sometimes is so much developed as to meet the advanced or rostral 

 portion of the carapace, and thus enclose the first or ophthalmic somite within a 

 channel. In such cases the ophthalmic somite frequently ceases to be a calcareous 

 structure, and thus gives colour to the opinion held by many, among whom Claus and 

 Fritz Miiller J are the highest authorities, that the ophthalmopoda have no ocular somite, 

 and therefore are not homotypical of the limbs attached to the other somites among the 

 Arthropoda. 



The ophthalmic somite as a distinct and limb-bearing segment is capable of being 

 determined in several separate genera throughout the Crustacea, as, for instance, in 

 Squilla, as shown by Milne-Edwards in his Histoire des Crustaces, and in Palinurus 

 vulgaris. 2 In Cancer pagurus the ophthalmic somite exists distinctly separated from 

 the others, but is enclosed as a calcareous bar, and hid within the first, or anterior, 



1 Facts and Arguments for Darwin, English Translation, p. 14, note 1, 1869. 



2 Brit. Assoc. Advancement of Science, 1877, Report on the Present State of our Knowledge of the Crustacea, 

 pi. ii. fig. 8. 



