28 REPORT — 1855. 



the AmjjJiipoda^ there are four so constructed as to form a tail. Of these 

 four, tliree pairs are arranged upon the same tj'pe ; the other, which is the ex- 

 tremity, or twenty-first ring, can only be contemplated in the character of au 

 obsolete segment with its rudimentary appendages. 



Thus the segments which form the abdomen support three distinct forms 

 of appendages. Three anterior are constructed upon one type, three suc- 

 ceeding upon a second, and the last, which for convenience we shall de- 

 signate by the name of Telson (from reXo-oj', extremity), upon a third ; or, 

 perhaps to speak more correctly, it is a rudimentary appendage, modified 

 upon the type of the preceding three. 



Thus we perceive a singular coincidence, that the most anterior as well 

 as the most posterior segments of the animal are annihilated and represented 

 by their respective appendages only, a circumstance which appears to reverse 

 the law in embryological development in this class of animals, where we find 

 that the earliest developed parts are the anterior and posterior extremities of 

 the animal, the intermediate segments being the result of subsequent growth. 



Having compared the twenty-one segments of the crustacean type with 

 those of the Amphipoda, it will next be desirable that we should see to what 

 extent the separate parts or appendages may or may not differ from those in 

 the other forms. 



Organs of vision. — The first normal segment of the typically perfect 

 Crustacea is represented in the Amphipoda by its appendages only ; the eyes, 

 which appear to be lodged between the two pairs of antennae, are homolo- 

 gically anterior to the antennae, and are supplied with nerves which are the 

 most anterior pair given off by the cephalic ganglia. 



In the higher orders the eyes are projected upon footstalks. In \}tie, Amphi- 

 poda they are sessile. This distinction between the two has been thought 

 by naturalists generally to be an important signification in relation of one 

 tribe to that of the other; hence the feature has been made available as a 

 demarcation of distinct orders, it being taken for granted that so visible an 

 alteration in these organs must be accompanied by considerable and im- 

 portant changes in other parts of the structure. 



The eye in relation to the typical animal must be viewed as an appendage 

 of the first normal segment peculiarly developed to perfect its adaptation for 

 the fulfilment of certain requisite conditions; alter the same manner, the 

 mandibles, chelae and feet are necessary forms for other uses. 



In the Brachyura an ocular appendage consists of two articulations, at 

 the extremity of which the eye is lodged, in the same manner as we might 

 presume the hand would hold a ball, or, to give a more correct idea, be 

 developed into a ball having power of vision. 



It appears to be a law in the decreasing structural importance of Crustacea, 

 that the segment supporting the appendages shall disappear before the appen- 

 dages that it supports ; thus in Macroura the segment has disappeared, but 

 the eye is still borne on. footstalks. In the Amphipoda it appears that the eye 

 alone remains ; the segment and the articulating portion of the appendage 

 not being developed, the eye is presented so deeply within the segment suc- 

 ceeding, that it appears to be behind the antennae. But its position, wherever 

 situated, can only be to meet peculiar advantages under certain conditions. 

 Thus in the genus Taliirus the eye appears to be nearly at the top of the 

 head, while in Erichthoneus and some of the Podocerides it is carried upon 

 a projecting inferior angle, which in some genera of this subfamily is con« 

 siderably developed in advance of the head ; in which position, iu con- 

 sequence of the insufficient depth of structure, the eye projects upon the 

 internal surface, where it is lodged in the form of a protuberance. 



