THE EVOLUTION OF THE CRUSTACEA. 1 43 



matopocla, whose true representatives have been found in the Jurassic. The Isopoda seem 

 to be the only -group of Malacostraca not readily connected up with the Leptostraca. Their 

 depressed form, their sessile-eyes, and their antiquity all combine to indicate a separate origin 

 for the group, and it has already been pointed out how readily they can be derived directly 

 from the trilobite. 



While the Copepoda seem to have been derived directly from the Hypoparia, the re- 

 mainder of the Crustacea apparently branched off after the compound eyes became fully 

 developed, unless, as seems entirely possible, compound eyes have been developed indepen- 

 dently in various groups. Most Crustacea were derived from crawling trilobites (Lower 

 Cambrian or pre-Cambrian Opisthoparia), for they lost the large pygidium, and also the 

 major part of the pleural lobes. In all Crustacea, too, other than the Copepoda and Ostra- 

 coda, there is a tendency to lose the exopodites of the antenna?. 



These modifications, which produced a considerable difference in the general appearance 

 of the animal, are easily understood. As has been shown in previous pages, the trilobites 

 themselves exhibit the degenerative effect on the anterior appendages of the backward move- 

 ment of the mouth, and the transformation of a biramous appendage with an endobase into 

 a uniramous antenna is a simple result of such a process. The feeding habits of the trilo- 

 bites were peculiar and specialized, and it is natural that some members of the group should 

 have broken away from them. In any progressive mode of browsing the hypostoma was a 

 hindrance, so was soon gotten rid of, and the endobases not grouped around the mouth 

 likewise became functionless. The chief factor in the development of the higher Crustacea 

 seems to have been the pinching claw, by means of which food could be conveyed to the 

 mouth. It had the same place in crustacean development that the opposable thumb is be- 

 lieved to have had in that of man. 



An intermediate stage between the Trilobita and the higher Crustacea is at last exhibited 

 to us by the wonderful, but unfortunately rather specialized Marrella, already described. 

 It retains the hypostoma and the undifferentiated biramous appendages of the trilobite, but 

 has uniramous antennae, there are no endobases on the coxopodites of the thoracic appen- 

 dages, the pygidium is reduced to a single segment, and the lateral lobes of the thorax are 

 also much reduced. Marrella is far from being the simplest of its group, but is the only 

 example which survived even down to Middle Cambrian times of what was probably once 

 an important series of species transitional between the trilobites and the higher Crustacea. 



In this theory of the origin of the Crustacea from the Trilobita, the nauplius becomes 

 explicable and points very definitely to the ancestor. According to Caiman (1909, p. 23) : 



The typical nauplius has an oval unsegmented body and three pairs of limbs, corresponding to the anten- 

 nules, antennas, and mandibles of the adult. The antennules are uniramous, the others biramous, and all 

 three pairs are used in swimming. The antennx may have a spiniform or hooked masticatory process at the 

 base, and share with the mandibles which have a similar process, the function of seizing and masticating the 

 food. The mouth is overhung by a large labrum or upper lip and the integument of the dorsal surface of 

 the body forms a more or less definite dorsal shield. The paired eyes are as yet wanting, but the median eye is 

 large and conspicuous. 



The large labrum or hypostoma, the biramous character of the appendages, especially 

 of the antennae, the functional gnathobases on the second and third appendages, and the 

 oval unsegmented shield are all characteristics of the trilobites, and it is interesting to note 

 that all nauplii have the free-swimming habit. 



The effect of inheritance and modification through millions of generations is also 

 shown in the nauplius, but rather less than would be expected. The most important modifi- 



