154 INVERTERRATE MORPHOLOGY. 



hooks and followed by a varying number of segments or pro- 

 glottides, each possessing a set of reproductive organs and cap- 

 able of separating from its fellows, maintaining for a time an 

 independent life. The progiottides towards the hinder end 

 of the chain or strobila are the most advanced in development, 

 and one after another drop off and pass to the exterior of the 

 host's body with the faeces; more anteriorly the progiottides 

 are sexually immature, and still nearer the scolex they are to 

 be found in various stages of formation. In fact the hinder 

 end of the scolex rna}* be regarded as a zone of growth, new 

 progiottides being successively formed at this region. The 

 process of proglottid formation resembles not a little what 

 has been described as the non-sexual reproduction of the Dis- 

 comedusse, the scolex corresponding to the parent Scyphos- 

 toma and the progiottides to the Ephyne, the entire aggre- 

 gation in both cases being termed the Strobila. 



The exterior of the body of a Cestode is formed by a cuti- 

 cle without any trace of cellular structure, and is perhaps to 

 be regarded as a basement-membrane, the ectoderm, originally 

 present, having disappeared. The cuticle varies much in 

 thickness, and is throughout traversed by fine pores which 

 allow of the absorption into the body substance of the nutri- 

 tive fluids in which the Tapeworm lives, either directly or 

 by permitting the passage to the exterior of fine protoplasmic 

 processes from the subjacent tissue. Special developments 

 of the cuticle in the form of chitinous hooks are frequently 

 present, arranged in some Tsenias, for example, in a double 

 circle upon a prominence, the rostellum, at the apex of the 

 scolex, and forming a very efficient means of attaching the 

 worm to the wall of the intestine of its host. Beneath the 

 cuticula there is to be found a very thin muscular layer, the 

 peripheral musculature, but the main bulk of the muscula- 

 ture consists of those fibres which traverse the parenchyma. 

 These, especially the longitudinal and transverse ones, are 

 massed into strong bands, the former lying usually exterior 

 to the latter, and both enclosing a central mass which is trav- 

 ersed by weaker bundles of dorse-ventral muscles, and con- 

 tains the reproductive apparatus. 



In connection with the muscular system may be men- 



