PARKER : LONGITUDINAL FISSION IN METRIDIUM MARGINATUM. 49 



where the species was represented by thousands of individuals in easily- 

 accessible positions. The search was facilitated by the fact that often a 

 given pile or plank was covered with individuals of almost exclusively 

 one type of coloration ; almost all in one restricted location were orange- 

 colored, or brown, or whitish, etc. This is doubtless due chiefly to re- 

 production by fragmentation from the edge of the pedal disk, a non- 

 sexual process which insures to the offspring the minute characteristics 

 of the parent. My object was to find pairs of animals individually sepa- 

 rate, but situated next each other and strikingly similar in color and 

 markings, though entirely unlike those surrounding them. Such in- 

 stances, if found, would afford strong evidence in favor of the actual 

 occurrence of longitudinal fission. A search of some three hours' dura- 

 tion, in which between two and three thousand specimens were in- 

 spected in their attached positions, resulted in the discovery of six such 

 natural pairs. These were pairs which fulfilled the requirements of the 

 case in every respect, in that they were composed each of two animals 

 strikingly similar in size and coloration, and entirely isolated from others 

 of a like kind by a surrounding group of individuals unlike them. "What 

 seemed to be natural pairs of this kind were often met with in other 

 places, but as it was important to accept only pairs that were well iso- 

 lated, and as these would naturally not occur frequently, it is not sur- 

 prising that only six such pairs were found. 



The six pairs were placed each in a separate bottle and transferred to 

 the laboratory for further study. On examination all proved to be mono- 

 glyphic except one individual, which was diglyphic. The arrangement of 

 the complete mesenteries is indicated as follows : — 



D-2-3-4-5-6-7. 



First Pair D . 2 - 3-4-5-f -7-8. 



(D-2-D-4-54-7. 

 Second Pair j D-2-3-4-5-6-7. 



[D-2-3-4-5-6-7. 

 Third Pair D . 2-3-4-5- 6-7. 



Fourth Pair 

 Fifth Pair 

 Sixth Pair 



{S: 



D-2-3-4-5-6-7-8-9. 



D- 2-3-4-5-6. 



2-3-4-5-6. 



2-3-|-5-6. 



D- 2 -3- 4-5. 



D-2-3-4-5-6. 



The first pair were found attached to a large mussel shell, and were 

 killed and hardened without being disturbed from their natural posi- 



