44 REPORT — 1875. 



mined by their relationship as the first, second, and third pair of maxill(e,ov, 

 as Professor Westwood has suggested, siagnopoda. 



The appendages of the second division, or seven pairs of legs attached to 

 the pereion, may be readily denominated the pereiopoda ; but the anterior 

 two pairs are commonly variated for different purposes. In Brachyura they 

 fulfil the purposes of opercula to the mouth ; in the Squillidae and Edrioph- 

 thalmic Crustacea they are adapted for prehensile and ambulatory purposes ; so 

 that it may be found convenient to recognize them by a distinctive name, as 

 gnatliopoda. 



The appendages of the third division, or pleon, are never developed for 

 walking of prehension, but almost universally are formed for swimming ; and 

 even in the Isopoda, where these are utilized as branchial organs, they oc- 

 casionally fulfil the office of swimming-appendages. Not unfrequently the 

 last two, as in the Macrura, and the last three, as in Amphipoda, are variated 

 in form so as to enable the animal to spring when on land or dart a con- 

 siderable distance in the water ; and the term uropoda has been applied to 

 them ; but their variation is so inconstant that the advantage of defining 

 them by any special name will be less than the convenience arising from 

 the distinction. 



The integumentary structure is one of the most important in the Crustacea, 

 and a knowledge of the variations of its several parts is of much assistance, 

 not only to the student of the history of these animals, but also for elucidating 

 the knowledge of those forms that have passed away and can be studied only 

 through the impressions left imbedded in the rocks. 



The external skeleton of a crustaceous animal consists of series of rings, 

 that appear to repeat each other, differing only in modification according to 

 the necessity of the various portions of the animal. These rings represent 

 and protect externally various segments of the body, each division supporting 

 one pair of ai^pcndages only and the internal structure that relates to them. 

 Each of these several divisions we call a " somite," a term suggested, I 

 believe, by Professor Huxley in his lectures at the Royal College of Surgeons. 

 Of these there are never more than twenty-one ; and this may be considered 

 as being the normal number in all Crustacea above those known as the Ento- 

 mostraca, in some few of which, as in the genera Apv^ and Stegocepilialns, the 

 number of somites appear to be much more numerous ; but there the somites 

 appear to be repeated without having any function to fulfil or appendage to 

 support — a numeiical repetition only, the result of an enfeebled force. 



The first somite supports and carries the organs of vision. In some of 

 the most condensed forms the eyes are implanted on the outer side of the two 

 pairs of antennae ; but the internal structure invariably shows that the most 

 anterior pair of nerves are those that are connected with these organs. The 

 progress of development which we purpose alluding to in its proper place 

 clearly demonstrates the eyes to be the most anterior of aU the organs. 



The second somite bears the first pair of antennte, which, from its position 

 in the higher Crustacea, is generally called the inner pair, and from its posi- 

 tion in the lower forms is called the upper pair of antennae. 



The third somite supports the second or posterior pair of antennae ; this, 

 from its relative position to the other antennae in the higher and lower forms 

 of Crustacea, has been called respectively the outer and lower antennae. This 

 somite is so closely associated M'ith the fourth that it is not certain that they 

 exist distinct in any species of Crustacea. 



The three anterior somites are generally closely blended together. In the 

 earlier forms of development they are invariably so ; but in Squilla and its 



