i 



140 COMPAEATIVE ANATOMY. 



ventral groove ; the aciculi of tlie Echinorliyncliij and lastly, the 

 hooks of the Cestoda, which are in many arranged in a crown 

 (Figs. 59, 60), or placed in the wall of four protractile tubes 



(Tetrarhynchus). These be- 

 gin as thickenings of the 

 cuticle, but they form an in- 

 termediate step towards the 

 second group, for as they be- 

 Fig. 59. Head of ^ i '^ "' come chitinised they sink 



Tosnia ccBnurus „7,.j„-n;f down into the matrix, and 



(vesicle-fonn : Coenu- Fig. 60. aOcrfeUit- .-i-i t 



rus cerebralis seen ferent hooks from the still deeper. 



from in front). The crown of hooks of the In the seCOnd group the 



four suckers and the same form. Repre- ^^^^ ^^ aciculi nO longer arise 



crown of hooks in sentmg stages of de- i . • • i 



the midst of them velopment (after V. on the SUrtace, but in special 



can be seen. Siebold). depressions, which may be 



very well compared with 

 glands. The secretion is formed from one or more cells, and gets 

 a definite form as it becomes gradually chitinised ; varying in 

 different regions of the body. As a rule seta? are first formed 

 when metameres are. These structures vary greatly in size and 

 form, and are very different in the various genera and species. 

 With the exception of the Hirudinea they are found in all the 

 Annulata. They are almost always arranged in tufts (cf. Fig. 58, s), 

 two or four of which are connected with the parapodia of each 

 metamere. They function partly as locomotor organs, working 

 like oars in the swimming forms (Vagantes) ; when they are meta- 

 morphosed into hooks they may serve as seizing or clenching 

 organs (Tubicolte). They are best developed in the Aphroditidse, 

 where some of the finer setje form a felted layer, which covers the 

 back and elytra. 



The '' rod-like bodies " in the integument of the Turbellaria are 

 special structures, as are the similar structures in the Annelides ; in 

 many cases they call to mind the "urticating capsules of the 

 Acalephae.^' 



§ 110. 



An organ, the function of which is still somewhat uncertain, 

 belongs to the category of differentiations of the integument ; 

 this is the so-called proboscis of the Nemertina. It forms a 

 tube, which is enclosed in a special sheath, placed above the enteron, 

 and is often coiled; this tube opens in the anterior part of the body 

 above the mouth, whence it can be protruded by eversion. Several 

 divisions can be made out in this tube, one of which has stylets 

 at its base — generally a larger stylet in the middle, and at 

 each side several smaller ones in special pouches, which are 

 sometimes regarded as reserve stylets, and sometimes as structures 

 of a supernumerary character. The portion of the tube behind the 

 stylets is glandular in character, and is provided with an excretory 

 duct, which is placed close to the stylet. A muscle which arises 



