62 Mr. J. Wood-Mason on a new Genus 



Astacidce*, and developed to such an extraordinary degree in 

 Carideous Crustacea, one or two small folds or impressions 

 between or upon the second and fourth joints being all that 

 remains of the antennal scale and of the rudimentary joint 

 that in Nephrosis corresponds to the movable spine of Astacus f. 



The flagella of the antennas are remarkably long and of 

 excessive fineness at their extremities. 



The basal joint of the antennules has its upper surface greatly 

 inflated, owing to the remarkable development of the auditory 

 organ to which, in most Podophthalmatous Crustacea at any 

 rate J, this joint gives lodgment; and the almost globular 

 appearance of the joint as seen from the side contrasts strongly 

 with the flatness of its upper surface in Nephrops or Astacus. 

 Of the two remaining joints of the antennulary peduncle, the 

 first is short and cylindrical, being less than half the length of 

 the last, which in Nephrops is short and equal to that which 

 precedes it. The peduncle terminates, in the usual manner, in 

 a double flagellum, the outer branch of which is conspicuously 

 stouter than its filamentous and cylindrical fellow, perceptibly 

 compressed, and thickly fringed below with short hairs along 

 its distal third. 



The epistoma is much the same as in Nephrops^ save that 

 its posterior edge is straight and presents two small tubercles, 

 which give it the appearance of being slightly roundly emar- 

 ginate in the middle. 



The external maxillipeds and the parts of the mouth in front 

 of them are identical in structure with those of Nephrops. 



The eyes are completely rudimentary, neither pigment nor 

 corneal membrane being developed : the peduncles, indeed, are 

 present ; but even these are short, subcylindrical, mere aborted 

 structures, concealed entirely from view by the stout base of 

 the overhanging rostrum ; in spirit they have become perfectly 



* The antennal scale in Astacoides escaped the notice of Guerin, who 

 founded his genus on its supposed ahsence. 



f There appears to be no doubt that the antennal scale is the repre- 

 sentative of the outer of the two appendages borne upon the protopodite 

 at an early stage of embryonic life ; and if the movable spine in Astacus 

 and its undoubted homologue in the antennae of Nephrops represent the 

 inner of these appendages, then must the three distal joints of the peduncle 

 with the flagellum be looked xipon, as Dr. Fritz Miiller looks upon them, 

 as a new formation (Neubildung) and no longer as being in serial homo- 

 logy with the five distal joints of the other appendages, e. g. of an ambu- 

 latory leg, which represent the endopodite, the exopodite being completely 

 aborted or represented at most, as Rolleston remarks, by the annular con- 

 striction on the ischiopodite. For the facts relating to the transformation 

 of the embryonic exopodite into the antennal scale of the prawn, pari 

 passu with the budding-out of the flagellum and the abortion of the endo- 

 podite, vide Fritz Muller's admirable essay on the development of the 

 Crustacea, entitled <Fiir Darwin,' p. 41, fig. 31. 



i The caudal ear of Mi/sis forms an exception to this. 



