Ma7iy-headed Bladder-woi'm. 137 



of the cestode many-jointed tape-worms whieli this C3"st might 

 have given rise to if it had found its way into the intestines 

 of a dog ; and it is therefore as truly a social animal, or rather, 

 a colony of animals in this its cystic, as it is in its cestode 

 form. Each of these heads is called by helminthological writers 

 a ^scolex/ and the sac upon which they have developed them- 

 selves, and upon which the remnants of the six hooks of the 

 embryo might be detected, is the result of the growth of such 

 microscopic embryo, or ' proscolex,'' as the one figured pi. xii. fig. 6, 

 when by the aid of its hooks it has bored its way from the intes- 

 tinal canal into the blood-vessels of its '^host.^ The 'scolices' 

 possess two sets of organs for adhesion upon their proboscis ; viz. 

 four suckers placed proximally to the sac, and two rows of hooks 

 placed near their free apical extremity. The heads themselves, as 

 well as the parent vesicle, are endowed with considerable contractile 

 power; a layer of muscular tissue existing in their walls by the 

 action of which the heads with their armature can be retracted as 

 in this Preparation, or protruded. The cystic stage of the bladder- 

 worm is passed in the organism of some herbivorous animal, and 

 ordinarily in the brain of the sheep ; and it has been shown by 

 actual and repeated experiments with dogs, that when the cystic 

 form of this Taenia is swallowed by them, its various heads will 

 develope in their intestines into cestode worms, attaching themselves 

 by their armed proboscides, and producing sexual hermaphroditic 

 segments, the so-called ' proglottides,' in the interval between the 

 remnants of the embryonic vesicle and the asexual adherent head. 

 The entire colony is called a ' strobile.' The asexual character of 

 its ' head' may remind us of the similar exclusion of the generative 

 organs from the anterior segments of Hirudo Lumbricus ; and the 

 successive repetition of the testes in nine segments, as described in 

 the former of those animals, bears a distant resemblance to the 

 successive antero-posterior development of sexual deuterozoids, as 

 presented to us in the "^proglottides' of the 'strobile' in a Taenia. 

 A process, however, all but identical with the budding off of sexual 

 zooids by an asexual ' head' or ' nurse' as seen in the cestoid 

 stage of the parasitic Taenia, takes place in Autolijtus (Grube), 

 Nereis ^'^rolifera (O. F. Miiller), Myrianida (M. Edwards), which 

 are Vermes of the most high!}' organized order of the Polychaeta ; 

 and the resemblance pointed out in the preceding sentence, as 



