HALF AN HOUR WITH SEA-MATS AND SQUIRTS. 149 



Fig. 79. 



ber, have a gullet, stomach, and intestines communi- 

 cating with the anal aperture. In them, also, we get 

 a simple, rudimentary heart ; and although there is no 

 real blood, as in the higher animals, this heart causes 

 the circulating fluid " to ebb and flow like the tide," 

 as Dr. S. P. Woodward describes it. In this way the 

 two ends of the heart are alternately arterial and 

 venous. The nervous system is represented by 

 a ganglion, or nerve-centre. The reproductive 

 organs, situated in a fold of the intestines, are male 

 and female, combined in 

 the same individual. The 

 embryos, or young, are 

 free-swimming animals, 

 with a long tail like a tad- 

 pole. To the celebrated 

 John Hunter is due the 

 discovery that these crea- 

 tures were nearly allied to 

 the mollusca, and old Aris- 

 totle whose zoological 

 researches modern science 

 has proved to be so far 

 ahead of his time, remarks 

 of them : " They are the 

 only kind of mollusca whose 

 whole body is enclosed in 

 the shell, and that shell of 

 a substance between true 

 shell and leather; it may be cut like dry leather. 



Ascidia mentula. 



