MUSEUM OF COMPARATIVE ZOOLOGY. 145 



a disappointing failure, is the transfer from the salt water and its added 

 alcohol to 50$ alcohol. If the animal is but partially narcotized, the 

 muscular contraction induced by the transfer will spoil the specimen. 

 If, on the other hand, it be left too long, the weaker parts of the body 

 wall, especially the upper smooth zone of the introvert, swell out quite 

 rapidly (through osmosis?), and not only the external form but the his- 

 tological elements as well are badly distorted. The golden mean be- 

 tween these two extremes yields specimens as excellent for histological 

 work as for the study of external relations. Material preserved in this 

 way may be well stained by all methods. Where any stain has been 

 of especial value in the study of particular organs or tissues, it will be 

 noted under the topic in question. In this place I wish to express my 

 thanks to Prof. E. Ehlers of Gottingen and to Prof. A. Dohrn of Naples 

 for past favors, and to Mr. A. Agassiz, Prof. E. L. Mark, and Prof. E. B. 

 Wilson for more recent kindnesses in supplying me with material for 

 this study. 



II. External Anatomy. 



Selenka ('83, p. 92) has given a full description of the external char- 

 acters of Sipunculus nudus. There are however numerous points of in- 

 terest which first appear in a well expanded specimen, and which deserve 

 especial attention. The body consists of a large posterior region covered 

 by the quadratic integumentary areas (Hautfelder) and of a portion 

 anterior to these, which is called the introvert. 



1. Introvert. 



This includes on the average one sixth of the entire length of the 

 animal, and has in general the shape of a truncated cone (Fig. 1), the 

 anterior base of which, only a little less in diameter than the posterior, 

 is surmounted by a wreath of .tentacles which nearly encircle the mouth. 

 This region is ordinarily found entirely, or for at least two thirds of its 

 length, invaginated into the following portion of the body, and is only 

 rarely seen extended. In the latter condition it measures from three to 

 four centimeters in length. The circular muscle bands, which are sepa- 

 rate in the posterior part of the body, are here fused into an unbroken 

 sheet of muscular tissue. The fusion .takes place abruptly, and causes 

 the immediate cessation of the integumentary areas (Hautfelder) due to 

 the banded musculature, thus fixing a definite posterior boundary to 

 the introvert. On the latter one can distinguish (Fig. 1) four regions : 



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