and on the Function of the Antenna in Crustacea. 39 



In the GalatheadcBy the development of the mandibular ring 

 shows a closer approximation to the Brachyura than to the Ma- 

 croura, in which latter the two antennal rings occupy but one-half 

 of the whole carapace, the mandibular ring furnishing the re- 

 mainder ; whereas the Pagurida assimilate closer in the deve- 

 lopment of the same portion to the Macroura, — a circumstance in 

 its position among Crustacea corroborated by the fact, that pre- 

 vious to their taking possession of the shell of the mollusca^ they 

 exhibit all the characteristics of a perfect Macroura. Here, when 

 the nervous centre commences its first tendency to separate into 

 numerous distinct but less important ganglia, we find those 

 rings which carry distinct organs of sense, and furnished by 

 nervous filaments from the cephalic ganglion, decrease in a rela- 

 tive proportion to the rest of the animal : this, which we see very 

 apparent in the Macroura, is carried to the greatest extent in the 

 Diastylid-ce, where the carapace is constructed almost wholly of 

 the mandibular ring, having but a small area in the centre 

 which bears the antennae. And more, the carapace extends pos- 

 teriorly so as to envelope only the Gnathopods ; the rest of the 

 thorax being complete in the development of each separate ring. 



Lower in the scale we find that the whole thorax, including 

 the Gnathopods, is perfect in its distinction from the cephalic ring, 

 which latter is so reduced in importance as to difi'er little in ap- 

 pearance from that of a single ring ; whereas consolidation still 

 remains, and embraces within the compass of this one ring the 

 whole of the seven anterior. 



But we have seen in the descending scale of nervous force the 

 rings which carry the organs of consciousness degenerate in im- 

 portance, and yield to a corresponding development of the man- 

 dibular ring ; this law appears still to be in force in the Amphi- 

 poda, the lowest type of the Macroura form, in which I am 

 inclined to believe that the mandibular ring represents the whole 

 of the upper portion of the cephalic articulation ; the anterior 

 three being so diminished in importance, that they are to be 

 found only in the perpendicular anterior wall of the head *, or 

 perhaps represented by their appendages only. 



Since the present paper was communicated to the Linnaeau 

 Society, the author has had the opportunity, through the 

 kindness of Mr. J. Lubbock, of perusing Dana^s great work on 

 Crustacea, and it is but just he should state, that the conclusions, 

 which careful and long-continued observation of the homologies 



* An example of which raay be seen in the manner in which the two 

 first joints of the external antennae are absorbed in the frontal asi)cct of 

 Talitra ; a circumstance overlooked by naturalists, who have invariably de- 

 scribed this genus as having but three instead of the constant five articu- 

 lations to the peduncle of the inferior antenna:. 



