Distribution of Marine Animals. 129 



so with the marine Polychreta and otlier Invertebrates 

 down to the Foraminifera — only arenaceous forms ot the 

 latter are more abundant in abyssal regions, and it is said 

 that no Brachyurous crustacean has been met with below 

 1000 fathoms [Canon Norman). 



Sir John Murray thinks that migration into the deep sea 

 took place from the mud-line (viz. about 100 fathoms), and 

 that there is little evidence, from the observations made in 

 the ' Challenger,' to show that the deep sea has been peopled 

 since the earliest geological times. The uncertainty on this 

 head, however, is apparent by the statement of Prof. James 

 Geikie that it was the absence of these abysses in early times 

 (Palsfiozoic) which enabled many forms to become cosmo- 

 politan. Murray, again, considers that the fauna of the deep 

 water is less ancient than that of many shores {Lingula and 

 Heliopora) and fresh waters {Ceratodas). In considering the 

 deep-water fauna, however, it is well to bear in mind the 

 difficulty of bringing the animals up for investigation. 



A brief glance may now be taken at the bipolarity of 

 marine animals as promulgated by PfefFer and Murray. The 

 latter, especially from his experiences in the ' Challenger ' 

 expedition, has put forward a strong claim on this head. 

 He is of opinion that there are a large number of identical 

 and closely allied species in the extra-tropical regions of the 

 northern and southern hemispheres, which, so far as known, 

 are not represented in the intervening tropics — even though 

 the climatic conditions as regards temperature are the same. 

 He thinks that the identical species now living towards both 

 poles, or their immediate ancestors, had a world-wide distri- 

 bution, which involves a nearly uniform temperature through- 

 out the whole body of the ocean (probably in Middle Meso- 

 zoic times) J and that as the poles cooled these animals were 

 drawn towards the equator. As we go back to the Palaeozoic 

 period, he affirms, the tropical zone of temperature slowly 

 widens. Murray further supports his theory by pointing 

 out that pelagic larvse are absent in the cold waters of the 

 arctic and antarctic regions; yet this may have been acci- 

 dental, and due to the depth at which the tow-nets were 

 used. Certainly the Sponges, Coelenterates (Zoophytes), 

 Echinoderms, Annelids, and Molluscs of these regions have 

 ciliated pelagic larvae. This bipolar theory has been opposed 

 by Ludwig for the Sea-Cucumbers, Ortmann for the Crus- 

 taceans, and D'Arcy Thompson generally, whilst many of the 

 appearances may be explained by the cosmopolitan distri- 

 bution of the various types. 



In summing up, therefore, it would appear that the distri- 

 Ann. & Mag. iV. Hist. Ser. 7. Vol. xiii. 9 



