124 Prof. MTntosh on the 



fishes to certain areas, for their powers of progression in a 

 continuous element are great. 



The pelagic Tunicates, such as Salpa, Doli.olum i Pgrosoma } 

 and the Appendicularians, are practically cosmopolitan, 

 ranging from the northern seas to the Antarctic. Thus 

 Prof. Herdman found Sal pa runcinata fusiformis in water at 

 a temperature of 80° in the Gulf of Manaar, and the same 

 species occurs in the Antarctic seas. He has noticed that 

 some fixed forms, like Styela plicala, are also cosmopolitan or 

 range from the seas of Europe to those of Australia. lie has 

 also drawn special attention to the large size and the abund- 

 ance of the Tunicates in the Antarctic regions. Simple 

 Ascidians, again, are perhaps more common in shallow than 

 in deep water, and few extend to the abyssal zone. Compound 

 forms appear to attain their greatest development in the south 

 temperate zone. Botryllidic are partial (if not confined) to 

 the northern hemisphere. Distomidse are found in the 

 northern and southern hemispheres, whilst Polyclinidas are 

 southern (Herdman, 'Challenger'). 



In the present state of our knowledge it can scarcely be 

 said that the sea can be mapped into regions by the distri- 

 bution of the Ascidians, or that there is any clue to their 

 origin from pre-existing forms by their occurrence in modern 

 seas. Temperature has little influence on the distribution of 

 the simple forms, for they range from nearly freezing-point 

 upwards (Herdman), though, as pointed out in a former 

 Introductory Lecture, they are more conspicuous on the 

 seaweeds of the west than the east coast of Scotland. 



Out of fifty-eight families of marine mollusks forty-eight 

 are cosmopolitan, but the limitation of a whole family to an 

 area occurs very seldom. For example, while most of the 

 cones are tropical, AYallace points out that Pleurotoma is 

 cosmopolitan. In the same way the volutes are tropical, but 

 Mitra occurs in Greenland. The cowries are also charac- 

 teristic of warm regions, yet one species is found in Britain 

 and one in Greenland. Of the cuttlefishes some, like the 

 argonaut and pearly nautilus, are characteristic of warm 

 seas, whilst the majority are cosmopolitan, their enormous 

 numbers in the great oceans being only occasionally in 

 evidence by their destruction of fishes on the lines, by the 

 occurrence of their beaks in the stomachs of numerous fishes 

 (from the cod and Lampris to sharks), and by their forming 

 the chief article of diet for the sperm-whales. 



That the mollusks have had ample time to spread them- 



