548 THE MICROSCOPE. 



from the heart, may be traced through the larger branches 

 to the respiratory organs, consisting of branchial-fringes 

 placed above the mouth ; the blood may also be seen 

 returning tln-ough other vessels. The heart, a strong mus- 

 cular apparatus, is pear-shaped, and enclosed within a 

 pericardium or enveloping membrane, which is extremely 

 thin and pellucid. Affixed to the sides of the heart are mus- 

 cular bands of considerable strength, the action of which 

 appears very like the alternate to-and-fro motion occasioned 

 by drawing out bands of India-rubber, and which, although 

 so minute, must be analogous to the muscular cords of the 

 mammal heart; it beats or contracts at the rate of about 

 sixty times a minute ; and is placed rather far back in the 

 body, towards the axis of the shell. The nervous system 

 is made up of ganglia, or nervous centres, and distributed 

 throughout the various portions of the body. 



The singular arrangement of the eye cannot be omitted ; 

 it appears at an early stage of life to be within the tentacle, 

 and consequently capable of being retracted into it. In the 

 adult animal, the eye is situated at the base of the tentacle ; 

 and although it can be protruded at pleasure for a short 

 distance, it seems to depend much upon the tentacle for 

 protection as a coverlid — it invariably draws down the ten- 

 tacle over the eye when that organ needs protection. The 

 eye itself is pyriform, somewhat resembling the round 

 figure of the human eye-ball, with its optic-nerve attached. 

 In colour it is very dark, having a central pupillary-open- 

 ing for the admission of light. The tentacle, which is cylin- 

 drical in the young animal, becomes flat and triangular in 

 shape in the adult. The young animal is for some time 

 without teeth ; consequently, it does not very early betake 

 itself to a vegetable sustenance : in place of teeth it has 

 two rows of cilia, as before stated, which drop off when the 

 teeth are fully formed. The lingual band bearing the 

 teeth, or the " tongue," as it is termed, consists of several 

 rows of cutting spines, pointed with silica. 



It is a fact of some interest, physiologically, to know 

 that if the young animal is kept in fresh water alone, 

 without vegetable matter of any kind, it retains its cilia, 

 but arrest of development follows, and it acquires no 

 gastric teeth, and never attains perfection in form or size. 



