1901.] NATURAL SCIENCES OF PHILADELPHIA. 723 



appearance. The true caudicle is slender and thread-like, contrac- 

 tile and usually twisted; the regenerating papilla is stouter and 

 rod-like, and does not twist and contract like the caudicle. C. B. 

 Wilson (1900) contrasts the two structures as follows. On p. 

 116, alluding to the regenerating end, he says: " Such a papilla 

 is slender and almost pure while in colour. At first it it is difficult 

 to distinguish it from the true anal papilla with which the body 

 normally terminates, but it may be recognized by the fact that it 

 always possesses a very broad base which fades gradually into the 

 body wall, while the anal papilla is narrow and ends abruptly at 

 the emargination. ' ' 



The papilla on the Cerehraiulus found by the writer was about 7 

 mm. long, light in color and rather rounded, not yet having assumed 

 the typical flattened shape of the body. At its posterior end a 

 short, but in all respects a true caudicle was borne. A cross sec- 

 tion through this true caudicle shows that it consists merely of a 

 thin body wall enclosing ia central blood space. The body wall is 

 composed of the epidermis, in which the two lateral nerves lie, and 

 of the circular and inner longitudinal muscle layers. The blood 

 lacuna has no definite lining, but is bordered by numerous mesen- 

 chym cells. From the caudicle of Cerebratulus lacteus, like that 

 of Zygeupolia, the alimentary canal, the gonads and the rhyn- 

 choccel are absent. 



A cross section through the regenerating papilla of my Cerebra- 

 tulus has the same structure that is found in the section figured by 

 Dr. Montgomery. The outer longitudinal muscle layer and the 

 alimentary canal are present, and three blood vessels — one dt^rsal 

 and two lateral — instead of the central blood lacuna. This proves 

 finally that the structure observed by Dr. Montgomery is a regen- 

 erating posterior end and not a caudicle. 



In both worms the end of the body tapers quite gradually into 

 the regenerating portion, and it would be difficult to say where the 

 old tissue ends and the new begins, as differentiation has evidently 

 gone on for some time in the anterior part of the new tissue. The 

 most posterior sections of Dr. Montgomery's worm show that a 

 true caudicle had likewise begun to form there, but had subse- 

 quently been broken off just at its base. It is seen from the meas- 

 urements of the two worms that there is an abrupt change in size 

 between the end of the body proper and the caudicle, the caudicle 



