ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 83 



that arc blind in their adult condition, the eyes are generally well developed 

 in the younger stages. 



The eyes are, independent of their value as organs of vision, of great im- 

 portance in the study of the natural arrangement of the various forms of 

 animals in the subkingdom. They vary in form and character from the 

 most incipient ophthalmic spot to the compound eye erected on pedestals ; 

 but whether single or compound, solitary or in pairs, their form and compo- 

 sition is generally so persistent with certain forms and characteristics of the 

 life and habits of the animals that the few exceptions to the general rule do 

 not preclude them from being au important and valuable means of arranging 

 . Crustacea. 



This was first appreciated by Leach, in 1815, in his Classification of the two 

 great divisions of these animals. He arranged them under the two great 

 heads of PoDOPnTHALiiiA and EDEiorHTHALMiA — or those Crustacea that in 

 their adult stage have the eyes elevated on peduncles or footstalks, and those 

 which have them sessile or Avithout any footstalk. To this general observa- 

 tion the exceptions are very few. Among some genera that inhabit subter- 

 ranean passages and live in the dark, the footstalks are so reduced in size that 

 they can only be said to exist theoretically, inasmuch as we find them well 

 exhibited in their young and early stage. We must therefore assume that 

 they have depreciated from their normal condition through adverse circum- 

 stances. On the other hand, among the Edriophthalmia we have the genus 

 Tanais with its compound eyes elevated on their own pedestals, differing from 

 the pedunculated form only in being rigid and incapable of movement. ; 



In the Podophthalmia the eyes are implanted at the extremities of appen- 

 dages that are supported upon a separate and distinct somite. 



In 1837 Prof. Milne-Edwards demonstrated this to be the case in the genus 

 Squilla ; in 185-1 he states, in his " Observations sur le Squelette tegumen- 

 taire des Crustaccs decapodes," Ann. des Sciences Nat. p. 254, which I have 

 since confirmed (fig. 7 ), that in the genus Palmunis (the Langouste) " I'anneau 

 ophthalmique est parfaitement distinct, et se pre'sente sous la forme d'une piece 

 sclerodermique impaire, courte et large, situec en avant du bord frontal de la 

 carapace, et au-dessus de I'anneau antcnnulaire. Les appendices ophthal- 

 miq'ues, ou tiges oculaires, naissent des deux extremites de ce segment, et se 

 composcnt chacun de deux articles : une piece que j'appeUerai hasophthahnite, 

 et une seconde, qui porte a son extremite la cornee transparente, et qii'on 

 peut nommer podophihalmite" 



Milne-Edwards in the same manner shows how in several species of Pali- 

 nurus the antero-median portions of the carapace project more or less com- 

 pletely over the ophthalmic ring, and so {I. c. p. 255) " par consequent, 

 ouvert a ses deux extremites laterales pour le passage des tiges oculaires, et 

 I'espece de cadre ainsi constitue autour de la base de ces tiges forme la por- 

 tion fondamentalc de I'orbite ou trou orhitaire." 



Thus the orbit in Crustacea is formed by the thii'd or second antennal 

 somite reaching over and coming into contact more or less perfectly with the 

 first antennal somite. The greater or less in degree the separation between 

 the second and third somite above the ophthalmic somite the more or less 

 complete is the orbit in Avhich the eye is protected. This varies in different 

 genera, and is very complete in the genus Cancer (PL II. fig. 9), where the 

 ophthalmic somite is enclosed entirely by the union without fusion of theantero- 

 dorsal projection of the posterior antennal somite with the anterior antennal 

 somite ; but, according to Mihie-Edwards, in the genus Palinunis this perfec- 

 tion of the orbit varies. In P. vulgaris (fig. 8) the ophthalmic somite is naked, 



g2 



