Life-history and Anatomy of tlie Appendiculate Distomes. 355 



swimming. Giesbeecht sent some of his material to Monticelli 

 (1891, p. 13), who, however, differed from him and Möbius as to the 

 identity of the worm and determined it to be the youthful form of 

 Apoblema (Distoma) appendiculatum Rud., the adult form of which 

 he had frequently found in the intestinal tract of Clupea alosa, 

 C. pilchardus, and other fish inhabiting the Bay of Naples. He re- 

 marked upon the wide geographical distribution of the worm, being 

 found, as it is according to these observations, in the Mediterranean 

 Sea and the North Atlantic Ocean, and it is of considerable biological 

 interest that it has also" appeared off" the American coast. A distrib- 

 ution so extensive is, probably, to be accounted for by the fact 

 that the first host of the parasite is a common Copepod and the 

 final host a widely distributed fish that feeds upon Copepods and 

 other small pelagic animals. 



A further discussion of the larval life of these worms will be found 

 under the head "General Considerations", near the end of the paper. 



Form and Size of the Worm. 



The form of the worm we are discussing is cylindrical, the cross- 

 section being usually a perfect circle; it is elongated, with tapering 

 ends. The length of the largest individuals observed is 0.6 mm and 

 that of the smallest individual, 0.198 mm. The width of the larger 

 individuals is 0.11 ram. The extended appendix in the larger individuals 

 has a length of 0.13 mm ; it has the same diameter at its base as 

 the body or trunk of the worm and tapers towards its extremity. 

 The opening of the excretory vesicle is at the extremity of the 

 appendix. When the appendix is withdrawn, the sac it forms com- 

 municates with the outside by an opening which is controlled 

 by a sphincter muscle and which is usually small (PI. 25, Fig. 2 ap.p; 

 PI. 26, Fig. 8). 



The morphological significance of the appendix will be discussed 

 under the head "General Considerations". 



The oral sucker of the worm is subterminal and has a diameter 

 in large individuals of 0,053 mm ; the acetabulum is in the forward half 

 of the body, 0.09 mm from the mouth, and has a diameter of 0.08 mm. 

 The genital opening is on the ventral surface of the body just beneath 

 the lip of the oral sucker (PI. 27, Fig. 12). iThe worm is without color. 



The Integument. 



The integumentary membrane or cuticula has an average thickness 



