Fig. 201.—Section through a piece of a Cœnurus cerebralis, with four cephalic invaginations in different stages of development. At the bottom of the invaginations the rostellum, hooks and suckers develop. (From a wax model.)
Fig. 202.—Median section through a cysticercus, with developed scolex at the bottom of the invagination. (After Leuckart.)
The rudiment of the scolex appears as a hollow bud, the cephalic invagination usually directed towards the interior of the bladder cavity; on its invaginated surface arise the four suckers, and the rostellum with the hook apparatus is formed in its blind end; we thus get a Tænia head, but with the position of the parts reversed (fig. 201). In many cysticerci the head rises up from the base of the cephalic invagination and is then surrounded by the latter. A more or less elongated piece of neck also develops, and even proglottids may appear, as in Cysticercus fasciolaris (the larva of Tænia crassicollis of the cat) of the Muridæ, a process somewhat analogous to that of Ligula, etc.
The period that elapses from the time of infection till the cysticercus is fully developed varies according to the species; the cysticercus of Tænia saginata requires twenty-eight weeks, that of T. marginata seven to eight weeks, that of T. solium three to four months, and that of T. echinococcus longer still.
Fig. 203.—Cysticercus pisiformis in an evaginated condition, with neck, fore-body and bladder, with excretory network in its wall. 18/1.
With one single exception (Archigetes) the larvæ do not become sexually mature in the organ where they have developed; they must enter the terminal host, a matter that is usually purely passive, the carriers of the larvæ or infected parts of them being usually devoured by other animals. In this manner, for instance, the larvæ (Cysticercus fasciolaris) found in mice and rats reach the intestine of cats; those of the hare and rabbit (C. pisiformis) reach the intestine of dogs; those of the pig (C. cellulosæ) are introduced into man; those of insects are swallowed by insectivorous birds; those of crustaceans are ingested by ducks and other water fowl; perhaps, also, the infection of herbivorous mammals is caused by their accidentally swallowing smaller creatures infected by larvæ. Indeed, the researches of Grassi and Rovelli have taught us that such an intermediate host is not always necessary; Hymenolepis murina of rats and mice in its larval stage lives in the intestinal wall of these rodents, and as a larva it passes into the intestinal lumen and develops into a tapeworm in exactly the same way as the larvæ of other species that reach the intestine of the terminal host by means of an intermediate carrier. Probably this curtailed manner of transmission also occurs in many other species. In some cases the larvæ actively quit the body of the intermediate host, as in the case of Ligula and Schistocephalus, which travel out of the body cavity of infected fish and reach the water, where they may be observed in hundreds in summer, at all events in some localities. The larval stage of Calliobothrium—wrongly termed Scolex—has been observed swimming free in the sea, and the scolices of Rhynchobothrium, without their mother cysts, have been observed free within the tissues of several marine animals. In any case there is almost always a change of hosts, even in the single-jointed Cestodes, for the larva of Caryophyllæus, which lives in fishes of the carp family, is found in limicoline Oligochætes, that of Gyrocotyle (Chimæra) in shell-fish (Mactra), and different conditions can hardly be possible for Amphilina. Archigetes alone becomes sexually mature in the larval stage, but the life-history of this creature is not well known, so that it is not impossible that the attainment of sexual maturity as a larva in invertebrates (Oligochætes) is perhaps abnormal, and somewhat analogous to the maturity of some encysted Trematodes.