HABITS, ETC., OF CEREBBATULUS LACTEUS. 117 



The process is very rapid under irritation, and may be 

 completed within a few seconds after the application of the 

 irritant. It may also take place at two or three points at the 

 same time. 



If the irritation be extreme the process may be carried so 

 far that the whole body is divided into very short fragments, 

 the majority of which contain but a single pair of ovaries or 

 testes. This is a frequent result when the worms die through 

 a stagnation of the water in which they are kept. 



In such instances the fission goes forward slowly and may 

 occupy several hours, and the contents of the genital pouches 

 are commonly discharged, whatever their state of maturity. 



Fresh water acts as a powerful irritant upon Cerebratulus, 

 and always induces dismemberment, but it does not kill very 

 quickly. 



Fission and regeneration in Cerebratulus is not confined to 

 the body-walls; it often takes place in the proboscis. 



Under the action of powerful irritants both the anterior 

 and posterior connections of the proboscis are severed at 

 once, so that the extruded organ is not turned inside out, but 

 remains in its ordinary condition, or it may be half everted 

 and then extruded bodily. In either case it often lives a long 

 time, many days or even weeks. If it be irritated, and some- 

 times voluntarily, it moves in such a lifelike manner, and 

 looks so much like a young worm, that it might easily be 

 mistaken for one. Indeed, a similarly extruded proboscis of 

 another species did mislead several excellent zoologists (6). 



They described it as a young animal, and its possessor as 

 viviparous, and presented both as such to the British Museum, 

 where they now remain as striking examples of the folly of 

 judging by external appearances. A proboscis thus extruded 

 is reproduced by the formation of a small conical papilla on 

 the surface of rupture just in front of the ganglionic commis- 

 sure. This papilla grows backward rapidly, and a new organ 

 is formed in a few weeks. A similar renewal takes place 

 when the proboscis is forcibly removed by cutting (cf. 33). 



This species, therefore, in common with others, possesses 



