164 COMPARATIVE ANATOMY chap. 



Medusa, the Scyphistoma. When this Soyphistoma has developed to a certain 

 stage, in one case tlie larger portion of the body tears itself from the stem as a free- 

 swimming Medusa. The remaining stem can, however, become regenerated into a 

 complete attached Medusa (monodisc strobila), and the whole process may be repeated. 

 "We thus have here multiplication by detachment and subsequent regeneration. 

 The detached piece has indeed so little to regenerate in it that the regenerative 

 process may be described as cicatrisation. 



Or again the stem of the Scyphistoma becomes regenerated into a new 

 Scyphistoma before the iirst Medusa has detached itself, and when this regenerative 

 process continues without the 3Iedusm at once fully detaching themselves we have 

 a polydisc strobila. "We call the whole process strobilation, and it has been described 

 as asexual multiplication by axial budding. The polydisc strobila is a temporary 

 animal stock. 



What has here been said helps us to understand the 



Organisation of the Cestoda Body. 



In the body of the large majority of Cestoda the seolex (Fig. 117) 

 is distinguishable from a row of subsequent segments or proglottides 



(Fig. 110, E, p. 153 ; Figs. 115, 116). 

 The small pear or cone-shaped seolex 

 itself consists of the head and neek. 

 The former carries the organs of 

 adhesion (suckers, hooks, proboscides), 

 by means of which it attaches itself 

 to the intestinal wall of the host. 

 In it lie the single commissures be- 

 tween the longitudinal trunks of the 

 Fig. 117.— Three heads of Tapeworms nervous system which may be regarded 



Srti4;°%'TBXfc:;/alus' - brain commissures. It therefore 

 latus. corresponds with the anterior end of 



the Trematoda body. The thinner neck 

 portion of the seolex is followed by the flattened segments, which are 

 small at first but increase in size posteriorly. The neck portion of the 

 seolex constantly produces new segments, which push back those already 

 existing. The oldest and largest segment of the whole chain is there- 

 fore the hindmost. In the segments the genital organs develop ; indeed, 

 the whole hermaphrodite genital apparatus of each segment answers 

 to the whole genital apparatus of a Tremaiode. The male genital organs 

 are first developed in each segment, then the female; then follows 

 fertilisation, and finally the segment is little else than a case which, 

 besides the remains of the genital organs, is almost exclusively occupied 

 by the extended uterus containing thousands of fertilised eggs. The 

 row of segments from the head to the last segment represents the row 

 of consecutive stages of development of the genital organs. The last 

 segments from time to time detach themselves singly, or several together, 

 and reach the exterior with the excrement of the host. 



On comparing the head and the segments we find that the head has 

 no genital organs, and none of the segments have the organs of adhesion 



