202 CRUSTACEA chap. 



actual identity of species iu several cases, e.g. Pachyclieles pana- 

 mensis and Hippa emerita, and the same thing has been observed 

 for the marine fish. 



Another connexion, at any rate during early tertiary times, 

 which probably existed between now isolated tropical coasts, was 

 across the Atlantic from the West Indies to the Mediterranean 

 and West African coasts. Numerous facts speak for this 

 connexion. Species of Paliniirus and Dromia occur in the West 

 Indies and the Mediterranean, which only differ from one another 

 in detail, and a connexion between these two regions has been 

 urged from the minirte resemblances of the late Cretaceous Corals 

 of the West Indies with those of the Gosau beds of S. Europe, 

 and also of the Miocene laud-molluscs of S. Europe with those at 

 the present time found in the West Indies. 



To account, then, for the present distribution of littoral 

 Crustacea we must imagine that great changes have taken place 

 during comparatively recent times in the coast-lines of the ocean, 

 but the guiding principle in both the past and present has been 

 temperature, and this factor enables us, despite the immense 

 changes in the configuration of the globe that must have taken 

 place, to divide the coasts latitudinally into Arctic, Antarctic, 

 and Circumtropical zones. 



Pelagic Crustacea belong chiefly to the Copepoda (Calanidae, 

 Centropagidae, Candacidae, Pontellidae, Corycaeidae), a few Ostra- 

 coda (Halocypridae and Cypridinae), and among Malacostraca a 

 few Amphipoda (Hyperina), some " Schizopoda," and among 

 Decapoda only the Sergestidae, if we except the few special 

 forms which live on the floating weeds of the Sargasso Sea, e.g. 

 the Prawns Virhius acuminatus and Latreutes ensiferus, and the 

 Brachyura Neptunus sayi and Planes minutus. Besides these 

 Crustacea which are pelagic as adults, there is an enormous host 

 of larval forms, both among Entomostraca and Malacostraca, 

 which are taken in the surface-plankton. 



In dealing with the Copepoda we have already mentioned the 

 vast pelagic shoals of these organisms which occur at particular 

 times of the year, and have an important influence on fishing 

 industries. A nomcdocera patter soni (Fig. 27, p. 60) is a good 

 instance of this. It is a large Heterarthrandrian, about 3 mm. long, 

 with the body of a fine bluish green colour ; it has a remarkable 

 power of springing out of the water, so that a shoal has the 



