PROCEEDINGS OF SOCIETIES. 197 



two ciliated ridges produced into well-marked processes. These 

 subsequently shrink up^ and the animal becomes a simple 

 Planaria ; the resemblance to some Echinoderm larvae in this 

 form is striking. In a Nemertes, a larval form which has 

 been named Pilidium is produced^ in the interior of which 

 the young worm develops, enclosing the alimentary canal of 

 the larva, and finally escaping from it, leaving the rest of the 

 larva to perish. This is identical with what goes on in some 

 Echinoderms. The Cestoidea are represented by the common 

 Tape- worm. When in its habitual haunts, the tape-worm is 

 quite an active creature, exhibiting considerable power of 

 movement. The head presents two rows of hooks and four 

 suckers. A circular vessel exists in the head from which 

 proceed four longitudinal stems, the branches of which are 

 ciliated ; they open together by a terminal pore at the last 

 joint, the canals of each joint being connected to those of the 

 succeeding joint by such a pore. It is said that a nerve- 

 ganglion exists in the head of Taenia, but this appears very 

 doubtful. In the integument are minute oval bodies, vari- 

 ously dispersed. They are the so-called " calcareous corpus- 

 cles," but are by no means always calcareous. It is suggested 

 that these corpuscles are at the extremities of fine branches of 

 the water-vascular system, and are composed of Guanin (an 

 eflFete product allied to uric acid), since such bodies have been 

 found in the vessels of Distomata, where Guanin also has been 

 detected. Each segment of the tape-worm is hermaphrodite, 

 and has its genital pore. The organs are arranged essentially 

 on the Trematod plan — a penis, testicular sacs, vagina, 

 ovarium, germarium, and great uterine chamber. The penis 

 has been continually seen to pass into the vagina of the same 

 joint, whence self-impregnation has been inferred but not 

 proved. 



Two hosts are not necessary for the tape- worm. A man 

 who swallowed the joint of a Tania solium would have the 

 eggs hatch in his stomach, and make their Avay into his 

 muscles. , There they would assume the hydatid form, and 

 when this man was eaten by another (for men were un- 

 doubtedly cannibals in the earlier periods), the head of the 

 hydatid would give rise to a tape- worm. Usually, uow- 

 a-days, the pig or ox hatch the tape- worm's eggs for us. The 

 larva has a bilateral symmetry, with three pairs of hooks. 

 On being carried by the blood into the muscular tissue it 

 assumes the pupal condition, developing into a large sac, in 

 an involution of which the head appears growing inwards 

 until by pressure it is forced inside out. The terms larva, 

 pupa, and imago may be fairly used in this case. The pupa 



