Classification of the Crustacea Malacostraca. 147 



we come to compare the characters (as given, for instance, by 

 Sars *) of the Euphausiidas on the one hand, with those of 

 the Mysidae, Lophogastridae, and Eucopiidaa on the other, we 

 find tliat, with one important exception, to be discussed 

 presently, the two groups do not agree in one single character 

 which they do not share with the lower Decapods, and for 

 the most part also with the Stomatopoda and Leptostraca. 

 They agree in possessing a carapace, movable eyes, a scale- 

 like exopodite on the antenna, an elongated and ventrally 

 flexed abdomen, and a " tail-fan " formed by the lamellar 

 rami of the last pair of appendages displayed on either side 

 of the telson. This combination of characters goes to make 

 up what might be called the caridoid " facies," and at first sight 

 strongly suggests affinity between the groups exhibiting it. 

 It seems reasonable to suppose, however, that these characters, 

 together with such others as the natatory exopodites of the 

 thoracic limbs, are precisely what we must attribute to the 

 hypothetical stock of the Malacostraca, and that the caridoid 

 form has been retained in each of the divergent branches 

 proceeding therefrom by those members which have adhered 

 most closely to the primitive habits of life, and especially of 

 locomotion. That the stalked eyes and the carapace are 

 primitive features is not now disputed, nor can it be doubted 

 that the possession of an exopodite on the antenna is also 

 primitive, though it has been lost by the Leptostraca. The 

 lamellar form of this exopodite is intelligible as an adaptation 

 to swimming habits, and its reduction or loss corresponds 

 fairly closely in most cases with diminished natatory powers. 

 The fan-like disposition of uropods and telson is another 

 character not shared by the Leptostraca, which, nevertheless, 

 was probably possessed by the primitive Malacostraca, since 

 it occurs in the lower Decapoda and the Stomatopoda, and 

 also, though more or less modified, in Cumacea and many 

 Isopoda. The retention of these primitive characters does 

 not necessarily imply any special affinity between the various 

 groups which exhibited them. 



The one character, above referred to, which is stated to 

 distinguish all Schizopoda from the Decapoda is the freedom 

 of the terga of one or more of the posterior thoracic somites 

 from the carapace. In the Mysidae, Lophogastridae, and 

 Eucopiidae at least five of these somites are complete upon 

 the dorsal side and distinct from, although more or less over- 

 lapped by, the carapace. It has been stated that in the 

 Eupnausiidse the last thoracic somite remains distinct, while 



* Rep. Schizopoda ' Challenger,' pp. 10 & 11 (188/5). 



10* 



