WANDERING OF THE HEADS OF TETRARHYNCHUS. 371 



this century. In such cases the free posterior end has usually a thick 

 fringe of hair-like spines, just as in the Tetrarhynchi the rest of the body 

 is frequently covered to a large extent with small closely set bristles. 

 Not unfrequently this separation of the head leads in course of 

 time to a complete emigration. Not only are Tetrarhynchus-bladders 

 frequently found forsaken by their inmates, but also Tetrarhynchus- 

 heads occur in places and in animals in which the other stages of 

 their development are sought for in vain. Of the numerous cases of 

 this kind (all those forms which Eudolphi originally enumerated in 

 his genus Tetrarhynchus consist simply of these isolated heads) I shall 

 only mention 1 the Tetrarhynchus-heads found in the muscles of Sepia 

 (T. macrobothrius ? Fig. 216), which sufficiently reveal their origin 

 by the hair-like fringe at the posterior end, and have already been 

 frequently observed by former investigators (Eudolphi, delle Chiaje, 

 Wagener, &c.). At one time they are found boring and digging 

 between the fibrous bundles of the mantle, and at another time quies- 

 cent, and then surrounded by a thin envelope of connective 

 tissue, but always at the same stage of development. 

 Only in size do they exhibit any marked difference, for 

 some have double the diameter of others, but this 

 amounts to nothing more than that these inmates have 

 remained in their host for a different length of time. 

 The latter are not, of course, to be regarded as the final 

 hosts. They play the part of intermediate hosts, and 

 only affect the life-history of our parasite, in so far as m FlG - 216. 



. J 1 . _. ., . J Tetrarhynchus 



they extend its distribution and facilitate or render pos- sepia ( x 12). 

 sible the subsequent transference. 



It also looks as though the Tetrarhynchi furnished by no means the 

 only example of such a change of hosts. 



Young unjointed Cestodes, not unlike isolated heads of Tcenice, 

 have long been known to inhabit the intestines of many sea-fish. 

 These have a so-called " frontal sucker," more or less highly developed 

 between the other four suctorial pits, and not unfrequently also two 

 red eye-spots behind the hook apparatus. Eudolphi founded a special 

 genus (Scolex) for these animals, and thought that they ought all to 

 be regarded as representatives of a single species (S. polymorphus). 



Thanks to the investigations of Wagener and van Beneden, we 

 have, however, become more thoroughly acquainted with these para- 

 sites. We now understand not only how to distinguish the different 

 species, but also know that these gradually change the originally 

 simple structure of their suctorial cavities for a more complicated 



1 According to a communication of v. Ihering, the genus Tethys may be added to the 

 number of the animals in which these parasites are known to occur. 



