a. Scolex. b. caudal vesicle. c. (in A) six embryonic hooks.
The next important change consists in the development of a head, which becomes the head of the adult Tænia. This is formed in an involution of the outer wall of the anterior extremity of the cystic worm. This involution forms a papilliform projection on the inner surface of the wall of the cystic worm, with an axial cavity opening by a pore on the outer surface. The layer of cells forming the papilla soon becomes divided into two laminæ, of which the outer forms a kind of investing membrane for the papilla. The papilla itself now becomes moulded into a Cestode head, which however is developed in an inverted position. The suckers and hooks (when present) of the head are developed on a surface bounding the axial lumen of the papilla, which is the true morphological outer surface, while the apparent outer surface of the papilla is that which eventually forms the interior of the (at first) hollow head. Before the external armature of the head has become established, four longitudinal excretory vessels, continuous with those in the body of the cystic worm, make their appearance. They are united by a circular vessel at the apex of the head. The development is by no means completed with the simple growth of the head, but the whole inverted papilla continues to grow in length, and gives rise to what afterwards becomes part of the trunk. The whole papilla eventually becomes everted, and then the cystic worm takes the form ([fig. 97]) of a head and unsegmented trunk with a vesicle—the body of the cystic worm—attached behind. The whole larva is known as a Cysticercus. The term scolex, which is also sometimes employed, may be conveniently retained for the head and trunk only. The head differs mainly from that of the adult in being hollow.
There are great variations in the relative size of the head and the vesicle of Cysticerci. In some forms the vesicle is very small ([fig. 98]), e.g. Cysticercus limacis; it is medium-sized in Cysticercus cellulosæ ([fig. 97]), and in some forms is much larger. The embryonic hooks, when they persist, are found at the junction of the trunk and the vesicle ([fig. 98] A, c). Though the majority of cystic worms only develop one head, this is not invariably the case. There is a cystic worm found in the brain of the sheep known as Cœnurus cerebralis—the larva of Tænia cœnurus, parasitic in the intestine of the dog—which forms an exception to this rule. There appears, to start with, a tuft of three or four heads, and finally many hundred heads are developed ([fig. 96] D). They are arranged in groups at one (the anterior?) pole of the cystic worm.
A still more complicated form of cystic worm is that known as Echinococcus, parasitic in the liver, lungs, etc. of man and various domestic Ungulata. In the adult state it is known as Tænia echinococcus and infests the intestine of the dog. The cystic worm developed from the six-hooked embryo has usually a spherical form, and is invested in a very thick cuticle ([fig. 96] E and F, and [fig. 99]). It does not itself directly give rise to Tænia heads, but after it reaches a certain size there are formed on the inner side of its walls small protuberances, which soon grow out into vesicles connected with the walls of the cyst by narrow stalks ([figs. 96] F and [99] C). In the interior of these vesicles a cuticle is developed. It is in these secondary vesicles that the heads originate. According to Leuckart, they either arise as outgrowths of the wall of the vesicle on the inner face of which the armature is developed, which subsequently become involuted and remain attached to the wall of the vesicle by a narrow stalk, or they arise from the first as papilliform projections into the lumen of the vesicle, on the outer side of which the armature is formed. Recent observers only admit the second of these modes of development. The Echinococcus larva, in addition to giving rise to the above head-producing vesicles, also gives rise by budding to fresh cysts, which resemble in all respects the parent cyst. These cysts may either be detached in the interior ([fig. 96] F) of the parent or externally. They appear to spring in most cases from the walls of the parent cyst, but there are some discrepancies between the various accounts of the process. In the cysts of the second generation vesicles are produced in which new heads are formed. As the primitive cyst grows, it naturally becomes more and more complicated, and the number of heads to which one larva may give rise becomes in this way almost unlimited.
Cysticerci may remain a long time without further development, and human beings have been known to be infested with an Echinococcus cyst for over thirty years. When however the Cysticercus with its head is fully developed, it is in a condition to be carried into its final host. This takes place by the part of one animal infested with cysticerci becoming eaten by the host in question. In the alimentary canal of the final host the connective-tissue capsule is digested, and then the vesicular caudal appendage undergoes the same fate, while the head, with its suckers and hooks, attaches itself to the walls of the intestine. The head and rudimentary trunk, which have been up to this time hollow, now become solid by the deposition of an axial tissue; and the trunk very soon becomes divided into segments, known as proglottides ([fig. 99] A). These segments are not formed in the same succession as those of Chætopods; the youngest of them is that nearest to the head, and the oldest that furthest removed from it. Each segment appears in fact to be a sexual individual, and is capable of becoming detached and leading for some time an independent existence. In some cases, e.g. Cysticercus fasciolaris, the segmentation of the trunk may take place while the larva is still in its intermediate host.
Fig. 99. Echinococcus veterinorum. (From Huxley.)
A. Tænia head or scolex. a. hooks. b. suckers. c. cilia in water vessel. d. refracting particles in body wall.
B. single hooks.
C. portion of cyst. a. cuticle. b. membranous wall of primary cyst. c. and e. scolex heads. d. secondary cyst.
The stages in the evolution of the Cestoda are shortly as follows: