392 



FAMILY TVENIAD.E. 



Specially characteristic are the varied peculiarities exhibited by 

 the organs of attachment. So distinctive are they, that one can, even 

 with a hasty glance, recognise the Tseniadae and distinguish their 

 different species. This is of course true, especially of the forms 

 armed with hooks, but even those with suckers only are readily 

 recognised. 



In details the armature varies exceedingly. The hooks which 

 compose it have a conical form, and have their points turned back- 

 wards ; but in size and number, shape and arrangement, they vary so 

 much and so strikingly, that almost every one of the nearly two 

 hundred species has its own characteristic appearance. Just to 

 suggest the range of these variations, I may mention that the number 

 varies from eight to several hundreds. Krabbe counted 360 in a 

 Tccnia from the emeu, and even 860 in a form found in the quail. 

 Their size varies from 04 mm. (T. crassicollis) to 0*01 

 mm., and even less, so that sometimes the embryonic 

 hooks may be larger than those of the adult. If the 

 number be small, then the hooks stand usually in a 

 single circle, but are as a rule in a double, or sometimes 

 even in a triple or quintuple ring. In many cases, the 

 double ring looks very like a single one, owing to the 

 minuteness of the intervening space. The hooks of the 

 second row alternate with those of the first, and are 

 usually distinguished by smaller size and different 

 shape. Sometimes the hooks of the posterior row differ 

 in number from the others. 



The hooks are attached by a sort of root, which is 

 sunk into the cuticle of the head. It is usually laterally 

 compressed, and produced forwards and backwards 

 into a more or less conspicuous process. The hinder is 

 always turned towards the crown of the head, and is 

 generally the longer, though in some species the reverse 

 holds true. In the angle between the two root-pro- 

 cesses, which of course belong to the same radius, the 

 above-mentioned rostellum is fixed, so that the hooks 

 FIG. 228. Lon- are> in a certain sense, seated upon the latter. 

 of^young^S This rostellum is the most important, if not the 

 scrrata, consist- on iy motor apparatus which the hooks possess, special 

 tirelyofheadaTd muscles attached to the root-processes being, as a rule, 

 neck. ( x 60.) wanting in the Tcenicc. The rostellum has the form of 

 a sometimes lenticular, sometimes oval or cylindrical bulb, of a more 

 or less powerful muscular character, and able, according to its form 

 and state of contraction, to exert pressure either on the posterior or 



