STRUCTURE OF PROGLOTTIDES. 85 



proglottides (Section I). With the first separation of segments, the 

 segment which hore the above-mentioned cicatrix, and which is also 

 usually characterised by its being aborted and more or less sterile, 

 is lost. We might perhaps more correctly say that this cicatrix- 

 bearing segment is therefore the first fragment which separates from 

 a mature tape-worm coloiw. On it, at least in the species which 

 behave like Siebold's cestoid worm from Avion, we should neces- 

 sarily find the embryonal booklets, if these were not too small. 

 The want of this aborted cicatrized segment is a proof that the 

 strobila has already given off fragments or segments. 



If we were to give a definition of the strobila, it is a 

 series of separate joints, individuals, which is produced from the 

 active scolex in the space between the part of the latter which 

 must be called the head par excellence and its citatrized ex- 

 tremity by a sexual reproduction, which remains in direct union 

 with the scolex which forms its point of support, and which, when 

 examined from the anterior to the posterior part, presents asexual 

 segments, half and fully-developed sexual segments and segments 

 engaged in sexual retrogression, of which the last have become 

 truly independent individuals. 



We have still to refer to the anatomical structure and to the 

 development both of the sexual organs and of the brood. 



As regards the structure, we find, in the first place, no digestive 

 apparatus in the whole colony, nor up to this time have nerves been 

 detected in it. 1 The most important points of structure in the tape- 

 worm colony appear to be a dissemination of calcareous corpus- 

 cles and the formation of the vessels. Of the vascular system of 

 the head we have already spoken under the development of the 

 scolex, and at the same time seen that a union of the four lateral 

 principal vessels takes place in the head ; that even in the head 

 smaller branches are given off from these main stems ; that these 

 vessels probably open outwards at the spot where the receptaculum 

 capitis (a portion of the so-called neck, as already remarked,) 

 passes over into the caudal vesicle, without uniting into a pulsa- 

 ting tube, as in the Trematoda; that these vessels certainly differ 

 in their arrangement from those of the Trematoda, which they 

 may resemble in their function ; and that the homologue of the 

 pulsating tube in the Cestodea may be placed, not at the hinder 



1 " A single ganglion has been discovered situated in the axis of the head, and 

 sending off nerves to the suckers in some Tseniadac." — Huxley. Ed. 



