GENERAL CONSIDERATIONS. 317 



Crustacea enables us to ascribe such a structure to the common 

 racial form. The anterior cephalic region comprised the frontal 

 organ (primary cephalic tentacle of the Annelida 1), the unpaired 

 (so-called Jl^auplius) eye, and the paired compound eyes, which we 

 must evidently assume to be inherited from the common racial 

 form. This development of the cephalic region and its limbs 

 gave those characters by which the Crustacea proper (primitive 

 Phyllopoda) were distinguished from the Palaeostraca and the other 

 Arthropodan stocks. In the primitive Phyllopoda the sexes were 

 probably separate ; they possessed a long dorsal blood-vessel with 

 segmental pairs of ostia, and perhaps also a pair of hepatic out- 

 growths in each segment. The presence of this last character is 

 supported b}' the organisation of the Stomatopoda {cf. on the 

 common racial form of the Crustacea, Lang's I'ext-hook of Com- 

 parative Anatomy, Vol. i., p. 406). 



In conchisioii ^ve iiuist briefly refer to the interrelationships of the different 

 groups of Crustacea. Among the Entoniostraca the Branehiopoda, among the 

 Mahxeostraca Nebalia, and in many points of inner organisation the Stomatopoda, 

 stand nearest to the jiriuiitive Pliyllopoda. Among the Entoniostraca, the 

 Copepoda were probably the first to branch off independently ; these while 

 undergoing, in adaptation to pelagic life, a certain degeneration (that of the 

 dorsal shield, the heart, and the respiratory organs, and the loss of the paired 

 eyes), in other respects, especially in the structure of the mouth-parts, retained 

 a very primitive condition. The other Entoniostraca (Phyllopoda, Ostracoda, 

 and Cirripedia) seem to stand somewhat nearer one another. Among the 

 Phyllopoda, the small Cladocera, consisting of few segments are evidently a 

 degenerate form of the Estheridae. For the Ostracoda we shall have to take as 

 a starting point a very primitive Phyllopod completely enclosed in a bivalve 

 shell, a form which thus must have resembled in appearance the Estheridae. 

 In the most primitive form among the Ostracoda, Cyjmdina, the structure 

 of the limbs clearly indicates a relationshiji with the Phyllopoda. Since we 

 were obliged to presuppose for the })riniitive Phyllopoda a body of many 

 segments, we shall have to assume for the Ostracoda a secondary diminution 

 in number of the segments. The Cirripedia also must be deduced from a racial 

 form similar to that of the Ostracoda, starting from the free-swimming Cypris- 

 like larva. According to Claus, a nearer relationship to the Copepoda must 

 often be assumed. This latter assumption rests upon the similarity of the 

 thoracic limbs, and the similar number of segments in the thorax of the two 

 groups. These features may, however, have been acquired independently in the 

 two groups, since they actually recur in other Crustacea (the Cladocera, for 

 example, having six thoracic segments), we cannot therefore consider the 

 question as decided. For instance, we do not forget that the t3^pical Copejjodan 

 characters (degeneration of the lateral eyes and of the dorsal shield, breaking 

 up of the second maxillae into two pairs of niaxillipedes) are not found in the 

 Cy2Jris larva of the Cirripedes. In judging of the systematic position of these 

 latter, we shall have to lay special stress on the presence of a large bivalve 

 shell from which the mantle of the adult develops. It therefore appears to 



