264 



THE MICROSCOPE. 



actively engaged near the side of tte egg, using all' its force to break 

 through the cell-wall, which it at length succeeds in doing ; leaving 

 the shell in the ova-sac, and immediately attaching itself to the side of 

 the glass-vase, to recommence the ciliary motion ; it then appears in 

 the advanced stage of life seen at /. It is still some months before it 

 grows to the perfect form represented at g, where the animal is drawn 

 with its sucker-like foot adhering closely to the side of the. glass-vase. 

 One of these animals will deposit from two to three of these ova-sacs 

 a week ; thus -producing, in the course of six weeks of two months, 

 from 900 to 1000 young, upon which the smaller kinds of fish feed. 



The shell itself is deposited in minute cells, that take up a cir- 

 cular position around the axis, on the u'nder-surface of which a hyaline 

 membrane is secreted, soon to become permeated with vessels ; at the 

 same time the integument expands, and at various points an internal 

 colouring-matter or pigment is deposited ; the point of one cell being 

 in contact with those of others, thus form the ribs seen in the shell.* 

 The increase of the membrane goes on until the expanded foot is 

 formed, the outer edge of which is rounded ofi' and turned over by a 

 condensed tissue having the form of a twisted wire, enclosing a net- 

 work of small vessels filled with a fluid in constant and rapid motion. 

 The course of the blood or fluid, as it passes from the heart, may be 

 traced through the large branches to the respiratory organs, con- 

 sisting of branchial-fringes placed above the mouth ; the blood may 

 also be seen returning through other trunks. The heart itself is a 

 strong muscular apparatus. It is ^ear-shaped, with a pericardium or 

 external wall- enveloping membrane, extremely thin and pellucid. Af- 

 fixed to the sides of the heart are muscular bands of considerable 

 strength, the action of which appears very like the alternate to-and-Jro 

 motion occasioned by drawing out bands of India-rubber, they being 

 analogous to the muscular cords of the mammal heart. The heart 

 beats or contracts at the rate of about sixty times a minute ; and it is 

 placed rather far back in the body, towards the terminal portion of the 

 shell. 



The nervous system is made up of many ganglia, or nervous cen- 

 tres, distributed through the different portions of the body, but con- 

 nected with each other by cords of communication ; the nerve-fibres 

 proceeding to the different parts of the body from the ganglia. 



The singular arrangement of the eye must not be omitted; it 

 appears in the early stage to be situated within the tentacle, and con- 



* See further researches by the author, published in the Microscopical Journal 

 1854. 



