408 COMPARATIVE ANATOMY CHAP. 



pair perhaps lie-longs to this category, because as mouth parts, generally provided 

 with masticatory processes, they serve not only with the others for locomotion, but 

 also for conducting food to the oral aperture. Where the egg is provided with a 

 somewhat richer supply of nutritive yolk, the larva which hatches from it may, as a 

 Mctaiuniplius, be provided with the rudiments of other limbs. 



The Xi/njilins, however, as typical Crustacean larva, certainly shows many 

 primitive Crustacean characteristics, such as the dorsal shield, the median eye, the 

 frontal sensory organs, the special form of the posterior antenna and mandibles 

 which, like the extremities which follow them, are developed as typical biramose 

 limbs. 



The Nauplius is thus to be traced back to a Trochophora larva, in which we 

 already find Crustacean characteristics; it is unsegmented, contains the rudi- 

 ments of the anterior cephalic portion of the adult Crustacean with the mouth, 

 and the rudiments of the most posterior end of the body with the anus. Between 

 the two lies an embryonic formative zone, from which, in the further development 

 of the larva, the rest of the body begins to form and becomes differentiated, as in 

 the Annelida, from before backward. The Nauplius is a typical Crustacean larva ; 

 the ancestors of the Crustacea did not as yet possess a typical Nauplius larva, 

 still less did they come from a Nauplius-like racial form. 



As for the Malacostraca, the testimony of comparative anatomy is unequivocal, 

 that the Lcptostraca not only stand nearest to the common racial form of this whole 

 sub-class, but also retain many primitive characteristics of the common racial form 

 of all Crustacea. The Leptostraca appear as genuine Malacostraca, first on account 

 of the formation of the regions of the trunk, which consists of a thorax of 8 segments 

 and an abdomen ; the latter, although it has one more posterior segment than the 

 typical Malacostracan abdomen, still carries the same number of pleopoda. Both 

 the oral and thoracic limbs show the typical Malacostracan character. The intestine 

 proves itself to be a Malacostracan intestine by its masticatory stomach, and the 

 special form of its hepatic glands, and the apertures of the genital organs have the 

 position which is characteristic of the Malacostmca. On the other hand, by the 

 possession of a bivalve shell - fold, the simultaneous development of the 8 free 

 thoracic segments and their appendages, by the rich segmentation of the nervous 

 system, and the elongated heart with many pairs of ostia, the Lcptostraca show them- 

 selves to be very primitive Mahicostraca, whose ancestors must have been racially 

 related to the Phyllopoda. 



The relationships of the other orders of Malacostraca are, according to the present 

 state of our knowledge, to be considered as follows : the Stomahyoda form an order 

 standing quite by itself, which, though showing further and somewhat peculiar 

 development (e.g. in the possession of gills on the pleopoda, sexual organs in the 

 abdomen, numerous hepatic appendages, special form of the thorax and its extrem- 

 ities) has still retained, in many points of its organisation, primitive characteristics, 

 as, for instance, the elongated dorsal vessel with many pairs of ostia, and the shell- 

 fold which leaves several thoracic segments free. 



Of the other Malacostraca, the Scliizopoda, and especially the Euphcvusidce have, 

 for the most part, retained characteristics belonging to the common racial form of 

 the Malacostraca. Of these the chief are the special form of the biramose thoracic 

 feet, their epipodial appendages which function as gills, the dorsal shell, etc. The 

 A rthrostraca have evidently a common origin with the Schizopoda. Among the 

 former the Anisopoda must occupy the most primitive place, as well on account of 

 the occurrence of a small dorsal shell as of that of small exopodites on the second 

 and third pairs of trunk feet (Apseudes). . 



The Arthrostraca are otherwise characterised by degeneration of the shell-folds, 



