68 



KNOWLEDGE 



[Mabch 1, 1898. 



antennip, there are some among them, as H. J. Hansen* 

 has shown, in which nineteen segments may be inferred 

 exactly comparable to the last nineteen of the Crustacea 

 Malacostraca. With the living forms of the male and 

 female Hdinmcnoi talprAites (Walker) (a and b), regarded by 

 Hansen as a wingless orthopterous insect, it can scarcely 

 be uninteresting to compare the species THpeltis tUjilodisrns 

 (Packard) (c) and Dipeltis rum' (Schuchert) (d\I fossils 

 derived from the lower Carboniferous system, and placed by 

 those authors among the entomostracan Apodidae. The 

 entomologists now, with some reason, claim these fossils 

 for their own, so that the common ancestors of insects and 

 Crustacea remain as heretofore the phantoms of an un- 

 discovered past. 



Reverting to our more immediate subject, a remark must 

 be made on the mandibular segment. Owing apparently 

 to that predominance which the jaw so often asserts in the 

 affairs of life, this segment, not content with its nineteenth 

 or other fractional share of the back, has spread itself in 

 an obtrusive and in what might be called an overbearing 

 manner. It assumes the title of carapace, or cephalotho- 

 racic buckler. It is no doubt a valuable shield, but, like 

 other saviours of society whose natural motto is " L'etnt, 

 cent moi," the carapace of the crab sometimes takes leave 

 to pose as if it were the whole animal. Of this an extreme 

 example is afJ'orded by the Californian Ciyptolitlmhs ti//ncu3, 



of which a portrait by 

 . ^'. Stimpson is here pre- 



.'^ ' ' , sented. As will be 



perceived, the great 

 shield, in dorsal view, 

 completely hides all 

 the working members 

 of the organism except 

 the little twinkling 

 eyes. 



The ten pairs of ap- 

 pendages which follow 

 the mandibles are objects of study of almost inexhaustible 

 interest, not only because of the variety of form and function 

 they exhibit in any one specimen, but because of the sur- 

 prising variety of that variety as we pass from group to group. 



CnipioUlhudifS tiipiriis. (Dorsal vi 



tion. It will be easily understood that this diversity of 

 function is matched by some diversity of form, and the use 

 of distinctive names, such as maxills, maxillipeds, and 

 trunk-legs, becomes indispensable. Some, in fact, are a 

 kind of jaws — organs of the mouth — while others are a kind 

 of arms or legs — organs of the trunk ; but the curious thing 

 is that the middle pairs may be either one thing or the 

 other, according to the group which owns them. The 

 term "maxillipeds," or jaw-legs, enshrines the idea that 

 these appendages, though used as jaws, are nothing but 

 modified legs ; and the corresponding term " gnathopods," 

 with the same meaning, hints at legs which are longing 

 to be jaws. The hypothesis we have to consider is that 

 all the appendages, including, with those already mentioned, 

 the six pairs belonging to the pleon, are modifications of 

 one original pattern. Between the primitive simplicity 

 to be expected of such a pattern and the complicated 

 structure observable in a crab's maxill*, it might at first 

 sight seem hopeless to find the requisite connecting links. 

 But extended comparison of features difficult to interpret 

 with those that are common and commonplace has long 

 ago brought out a sort of ground-plan of a crustacean appen- 

 dage. According to this it principally consists of a stem 

 and two branches. Three joints are perhaps the normal 

 number for the stem, but it often displays only two, and 

 occasionally only one. The joints of the branches are 

 indefinitely variable in number. But, limiting ourselves 

 for the present to the Malacostraca, it may be said that, in 

 the organs of the mouth and in the limbs of the trunk, 

 the inner or main branch of an appendage shows a 

 preference for not exceeding five in the number of its 

 joints. Add these to two in the stem, and entrust the 

 seven to the plasticity of nature, and then see what will 

 follow. A man has only to look at the noses and chins 

 of his friends and neighbours to know what may be 

 expected from modifications of shape and size. 



Imagine, then, a primitive limb of seven approximately 

 uniform joints. In all but the last of these room must be 

 found for the retractor and extensor muscles. For 

 firmness of attachment to the trunk it may generally be 

 convenient that the first joint should be short. The last, 

 which does not require muscles, may be thinner than the 



a. Talifnis. b. Porcellio. c. Crangoii. d. Primiio. e. Aura. f. Phronima. g. Sphi/rapus. h. Potamohius. i. Ati/a. 



Among the functions more or less generally allotted to 

 them may be reckoned those of tastmg and pasting, biting 

 and fighting, grasping and clasping, walking and a kind of 

 inarticulate talking, swimming, burrowing, house building, 

 besides the automatic services which they render to the 

 eggs in the brood pouch and to the animal's own respira- 



* " Contributions to the Knowledge of tlie Insect Fauna of 

 Camerun." Eiitomol. Tidsk-er., PI. II., Figs. 1, 2, 1894. 



t Proceedings U.S. JSational Museum, Vol. XIX., PI. LVIII , 

 Figs. 4, 6, 1897. 



rest. Being in frequent contact with external surfaces, it 

 may acquire a hardened apex and become claw-like, or, 

 for purposes of navigation, it may assume a broad, flat, 

 blade-Hke appearance. The other joints will certainly 

 not for ever maintain uniformity of length, and those 

 which are longer will at least sometimes have a pro- 

 portionate increase in breadth. By the course thus indicated 

 we arrive at the ordinary leg of an ordinary amphipod, 

 such as a sandhopper {see Fig. a), or that of an ordinary 

 isopod, such as a woodlouse {see Fig. i), or that of a 



