Evolution 



by the vertebrae, the bony segments of the backbone. Above 

 the notochord again lies the main nerve cord, a position which 

 it retains throughout the whole vertebrate group. The nerve 

 cord is simple in structure, with only a very slight swelling at the 

 front end, representing the brain. There are two main blood 

 vessels, an upper and a lower, which expand and contract alter- 

 nately throughout their whole length, and thus maintain the 

 circulation. The blood passes forward in the ventral vein, is 

 pumped through the fine vessels of the gills, and collected into the 

 upper artery. From this it is distributed throughout the body 

 by branch vessels, to be re-collected into the ventral vein. If the 

 reader will refer to the illustrations in Figs. 61, 62, and 63, 

 the relationships of these parts will be more easily understood. 

 There is a single small eye-spot, a single organ of smell, but no 

 hearing organs. It seems probable that this extremely ill- 

 developed condition of the sensory system is due in some measure 

 to degeneration, and is not a primitive characteristic. There are 

 numerous pairs of simple nephridia, which open into the peri- 

 branchial chamber, and bear a close resemblance to those of the 

 worms. 



The lancelet forms a most important link between the lower 

 and the higher animals. It is in all probability derived from some 

 form similar to Balanoglossus, and it certainly leads up to the 

 round-mouths, which form the next step in the ladder. 



Before describing these latter, however, we must briefly allude 

 to the highly remarkable group of the tunicates or sea squirts, 

 one of which is shown in Fig. 64. They are sedentary creatures 

 found attached to rocks or weeds on the sea floor, and in appear- 

 ance they remind one rather of misshapen potatoes than of higher 

 animals. They are in fact regarded by the fishermen who bring 

 them to the surface as plants, and they were for long looked upon 

 by zoologists as akin to the molluscs. The only definite external 

 features of the tunicates are two apertures at the upper end, one in 

 the centre and one somewhat on one side. The absence of any other 

 definite external characters is due to the fact that the creature is 

 enclosed in a mantle of cellulose. The central opening is the mouth, 



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