186 



ENTOZOA. 



any real advantage could accrue from a more complex disposition 

 of tlie liooksj but the mere fact of tlieir frequent variations in size 

 — ^witli constancy in the several species — shows that such arrange- 

 ments are by no means accidental. In the species just named, 

 however, the hooks are uniform in size, and arranged in spirally dis- 

 posed circles, carrying from twenty to thirty hooks each. In another 

 form, we find a marked irregularity both as regards the size and 

 disposition of the hooks. This is shown in the accompanying cut 

 taken from a drawing executed by Mr. Busk. In this figure it will 

 be noticed that the bases of the hooks are suddenly enlarged and 



Fig. 32. Portion of the proboscis of a Tetrarhynchus in the scolex condition ; from the abdomen 



of a whiting {Merlangus vulgaris) ; highly magnified. — Busk. 



implanted in the substance of the contractile tissue of the proboscis. 

 It is seldom that these root-like processes are so conspicuously 

 developed. The larva from which this fragment is taken was not 

 much larger than an ordinary pin's head, being enveloped by a 

 cyst precisely hke the one above described. Nearly aU the larval 

 Tetrarhynchi exhibit the cystic character so universal in the 

 scolex condition of ordinary tapeworms; occasionally, how- 

 ever, this cyst-like character is almost totally lost, the ento- 

 zoon remaining sexually immature and groping about with restless 



