22 



More commonly than not the eggs of the moUusca are laid in clusters on 

 bits of stick, or on seaweed, and, in the case of fresh water species, on the 

 leaves of aquatic plants. These often get carried by currents and floods to great 

 distances, where the eggs are hatched, and thus new regions may be peopled. 

 Many of the mollusca are free swimmers all their lives, such as the Cephalopoda 

 and Pteropoda, and nothing but the temperature of the water exists to prevent 

 their universal diffusion. 



Another class of molluscs, many of which float about free in the sea, some- 

 times in colonies and sometimes as single individuals, is the Ascidi^-eida. 



In many other ways, no doubt, can the diffusion of molluscs be accounted 

 for ; and, in the case of fresh- water species, wading birds perform no small part. 

 The eggs get attached to the feathers or the feet of the bird, and are carried by 

 it often long distances. Some of the species of the helix or snaU have a very wide 

 range, and most probably were diffused by the agency of birds. 



In this sub-kingdom we can observe, as before, that natural barriers separate 

 fa\mas ; for instance, the molluscs on the east and west sides of the Isthmus of 

 Panama are said to be quite distinct. Again, a different kind of sea bottom is a 

 barrier to some species ; thus the mytUus or mussel requires rocks, the solen or 

 razor-shell requires sand. 



These few cases out of many must suffice to show that, although the means ot 

 diffusion of molluscs are very ample, yet they also follow the general law of 

 peculiarity of distribution, though generally speaking the species of this sub- 

 kingdom have a wide range. 



On account of these differences in distribution, zoologists have been able to 

 define several provinces distingxiished by their different molluscs. 



"We must now take a rapid glance at the means of the distribution of the 

 sub-kingdom Annulosa. This sub-kingdom comprises the classes of crustaceans — 

 spiders, insects, myiiapods, worms, intestinal worms or parasites, andechinoderms 

 or star-fish. 



In this sub-kingdom we have examples of both terrestrial and aquatic animals. 

 Amongst the former are the spiders, insects, and earth-worms j and amongst the 

 latter the crustaceans, most ringed worms, the star-fishes, &c. 



In the aquatic classes the young larvse are, with hardly an exception, free and 

 able to roam about the water at pleasure, and in the vast majority of instances 

 the adult forms have the power of locomotion. 



These animals are found nearly all over the world, though many species, or 

 perhaps varieties, are very local, and, as we saw with regard to some sheU-fish, 

 they also require peculiar conditions of Ufe, such as depth of water, amount of salt 

 in the water, kind of sea-bottom, and above all degree of temperature. 



Amongst the terrestrial animals of this sub-kingdom are found some cuiious 

 and interesting cases of distribution. 



Although in the insect class the locomotive powers of the adult form are very 

 well developed as a rule, yet these animals are not very wide rangers, and species 



