72 NATURAL HISTORY OF OUR SHORES 



openings in a set of fine plates immediately surrounding the 

 anal opening at the apex. 



The eggs are very minute. When they are expelled 

 (on this coast it is usually in April) they furnish no further 

 appearance than a cloud of a brick-red, or sometimes 

 orange-yellow, coloration to the surrounding water. 

 Millions are set free by one individual. 



During the same period that of sexual maturity 

 spermatozoids, in a white cloud, are set free by the males, 

 and by the mingling of the products in the water fertilisa- 

 tion takes place. 



In the case of some of the tube-worms, presently to be 

 described, the same method of fertilisation takes place. 

 Captive specimens have been carefully watched by the 

 writer during this period, and it is curious to note how 

 the individuals of each sex, although far apart, become 

 excited. A sense must exist, which has not yet been 

 defined, and for which I will venture to coin a term. I 

 should call it " Telethesia" 



The changes that take place after fertilisation are very 

 rapid, within an hour after extrusion an egg becoming a 

 moving embryo. 



This process of fertilisation, and the subsequent changes 

 in the egg one of the most wonderful operations in Nature's 

 laboratory can be readily observed under the microscope, 

 the elements of the sea-urchin lending themselves to this 

 end more readily than any other subject. 



The " water- vascular system " I have mentioned as 

 operating to protrude the tube feet also performs other 

 functions in the animal's economy, but they are not well 

 known. 



The water in these vessels has communication with the 

 open sea through some openings in a specially raised and 

 modified plate on one side of the centre, and which is named 

 the Madreporiform tubercle. It is conspicuous in a living 



