560 CRUSTACEA. 



Decapod Crustaceans, especially in the Mesozoic and Tertiary de- 

 posits ; but the palaeontological interest of these is comparatively 

 small, and it will be sufficient here to give a brief sketch of the 

 general geological history of the three great tribes into which the 

 order Decapoda is divided. 



Order I. Cumacea. — This order includes small marine Crus- 

 taceans, in which there is a short carapace covering the head and the 

 anterior thoracic segments. The eyes are sessile, and the mouth-organs 

 resemble those of the Isopoda. The tivo aftterior pairs of legs at least 

 possess natatory exopodites, and the branchiae, are attached to the 

 epipodites of the maxillipedes. The abdomen is elongated and com- 

 posed of six segments, and a telson may be present or absent. 



The Cumacea are regarded by G. O. Sars as an order of Crustacea 

 related on the one hand to the Schizopods or Macrurous Decapods, 

 and on the other hand to the Isopoda. No fossil forms of the order 

 are known. 



Order II. Schizopoda. — This order includes the " Opossum 

 Shrimps " (My sis) and their allies, comprising small Crustaceans 

 which are in many respects related to the Decapoda. According to 

 G. O. Sars, the Schizopods occupy " the most primitive position 



Fig. 421. — " Opossum Shrimp " (Mysis oculata). m t Marsupial pouch. (After G. O. Sars.) 



within the division Podophthalmia, being apparently the least modi- 

 fied forms, in which the original characters distinguishing the pro- 

 genitors of the whole division would seem to exhibit least change." 

 The correctness of this view is shown by the fact that many of the 

 Podophthalmate Crustaceans pass in their development through a 

 " Mysis-stage," in which they present marked Schizopodous char- 

 acters. The Schizopods are characterised by the fact that the 



