212 



KNOWLEDGE, 



[September 1, 1900. 



Also among themselves they form three groups to which 

 nature has been pleased to give a rare distinctness. 

 These are known as the Gammaridea, Caprellidea, and 

 Hyperiidea, names derived from an early defined genus 

 in each, to wit, Gammarus for the fii-st, Caprella for the 

 second, Hyperia for the third. The middle set resemble 

 Manx cats and mankind and some other more or less 

 interesting creatui-es in the circumstance that they have 

 lost or nearly lost their tails, that pai-t of the body 

 which in the higher ci-ustaceans is decorously designated 

 the pleon. In the other two sets this part is rather 

 powerfully developed, almost always caiTving its proper 

 complement of appendages, namely, three pairs of pleo- 

 pods for swimming and three of uropods for jumping 

 or some equiv;deut mode of progression. The pleopods 

 show comjjaratively little variation, each usually consist- 

 ing of a stout stem to which are attached two branches, 

 many-jointed, and feathered with long setK which are 

 themselves feathered. The Gammai-ids and Hyperids 

 are distinguished one from the other by a character in 

 which the Gammarids agree with the Caprellids, a 

 character not of the tail but of the head. The maxilli- 

 peds in the Hyperiidea have only three joints, the re- 

 maining four having for some mysterious reason 

 vanished, whereas in the other Amphipoda some and 

 usually all of these joints are present, ^\^lether, there- 

 fore, by the head or the tail the three divisions of the 

 order are clearly distinguished and at the same time 

 closelv linked together. 



Piireipalpus linea. Mayer. From Mayer. 



The Caprellidea embrace two families, the Caprel- 

 lids and Cyamidae. The former of these are familiarly 

 spoken of as spectre-shrimps and skeleton-shrimps, and 

 in one of these species, the Parvipalpus linea, Mayer, 

 here figured, tenuity reaches perhaps its furthest amphi- 

 podan limit; nor does this jJrecious specimen make up 

 for want of breadth by any unnecessary length, since 

 it is only two-fifths of an inch long. It may be proper 

 to mention that in the lateral view, according to car- 

 cinological custom, the antenna; and legs of one side 

 only are portrayed. The third and fourth perajopods 

 are missing by accident. The first and second are 

 missing in natui-e, nothing of them remaining but the 

 little branchial sacs. Thus artistic convention, the 

 frailty of tlie specimen, and nature's thrift, have com- 

 bined to present the picture of a three-footed animal, 

 which nevertheless has ten feet by rights, and by classi- 

 fication belongs to a fourteen-footed order. Cousins to 

 the skinny clambering CapreUidffi, and closely resem- 

 bling them in stinicture, are the more sedentary 

 Cyamidee, commonly known as whale-lice. In appear- 

 ance these greatly differ from their near relations, 

 because, instead of being cylindrical and thread-like 

 with geniculating bodies, they are comparatively broad 

 and flat, adapted for close adhesion to the skin of their 

 gigantic hosts. The Caprellid* are ascetics, subduing 

 the flesh to such an extent that substance runs a risk 

 of passing into shadow. The Cyamidse, having adopted 

 a lethargic life in oleaginous luxury, seem to be gradu- 

 ally recovering some of that corporeal amplitude 

 which is . appropriate to the epicurean. 



The Hyperids are not quite so accessible as the other 

 two groups, being rarely found in England between 



tide-marks, except when thrown up by storms, either 

 independently or on the jelly-fishes, which some of them 

 frequent. Many of them are distinguished Dy the 



rrrnm.^ 





Teirathifrits moncoeuri, StebbiDg. 



extreme development of the eyes and by the glassy 

 transparence of the body. Often in the female the 

 second pair of the antenna; are obsolete. In the male 

 the same pair frequently show a curious arrangement. 

 They have delicatel)- slender joints of great length, 

 which, when not in use, can be folded together like a 

 cai-penter's rule and tucked away securely at the sides 

 of the animal. In some genera the folding plan is 

 carried further, being applied to the whole body, as in 

 the Tetrathyrus mottcteuri and Dithyrus faha here 

 figured. In Tttrafliyriis, " foul' doors." and Bithyrus, 



Dithyrus faha, PaEia. From Dana. 



'' two doors, ' allusion is made to the gi'eatly expanded 

 joints of the two pairs of legs which precede the tiny 

 last pair. It is a jjeculiar function for leg-joints to 

 have to assist in armoiu'-plating the owner's body. A 

 comparison of the two figures will show how the animal 

 by the infraventral folding of the broad joints in ques- 

 tion, and the apjiroximation of its head and tail, be- 

 comes a smooth little egg-like box, as compact a fortress 

 of its kind as could well be devised. Very different in 

 appeai'ance but not so verj' different in structui'e is ,the 

 C alamorhynchiis rigkluK, of which a dorsal view is given, 



Calamorhi/nc/iiis rigicliis, Stebbing. 



that does not show the rather insignificant legs. Its 

 generic name, meaning " pen-beak." is appropriate to 

 the form of the head in mature life, but it is born blunt- 

 headed. 



Of the three groups, the largest- publicity and promi- 

 nence has been obtained by the Gammaridea, and one 

 can easily believe that from this type the other two are 

 derived, because in each of those other two a feature 

 has become degraded or lost which the Gammaridea 

 still retain. The Gammai'idea alone of the three have 

 made their way into fresh water and on to dry land. 

 It is, indeed, a fresh water species, Gammarus pulex, 

 which the student has always at hand as a simple 



