[Illustration: FIG. 44.—Schematic longitudinal sections through the larvae of _Dipylidium caninum_ L. All these stages are passed in the body-cavity of the Dog-flea (_Pulex serraticeps_). (Compare Fig. 43 for further details.) A, Six-hooked larva with developing rostellum (shaded) and suckers (black). In this species the invagination (C, _invag_.) occurs after the formation of these organs, and not, as in most Taeniae, before it. B, Invagination commencing; the hooks are developing above the rostellum, while beneath it the nervous system (dotted) is seen. C, The invagination has now carried the suckers inwards. The tail has become distinct, and the whole larva at this stage is known as a Cysticercoid. _hk_, Larval hooks; _invag_, mouth of the invagination; _n_, central nervous system; _rost_, rostellum and hooks; _sc_, suckers, of which only two can be seen in a longitudinal section; four are really present. (After Grassi and Rovelli.)] {89}At one pole an invagination occurs, at the bottom of which the rostellum, suckers, and hooks are gradually formed, but inside out as compared with the head of the _Taenia serrata_. At this stage the larva (_Cysticercus pisiformis_) has usually issued from the liver and attached itself to the omentum. The invagination projects into the cavity of the bladder, within which a watery fluid accumulates. Thus the "bladder worm" is formed, the head of which is evaginated if the larva be introduced into the digestive system of a dog. The bladder and neck of invagination are digested, while the head, protected by these, remains, and forms the neck, from which the proglottides are afterwards segmented off. In _Taenia_ (_Hymenolepis_) _murina_ the whole development may take place in the parental host, the larva living in the villi, the adults in the cavity of the same rat's intestine (Grassi). The different forms of Cestode larvae depend largely upon the presence and degree of development of the caudal vesicle or bladder, which in _Scolex polymorphus_ (Fig. 38) (the young stage of _Calliobothrium filicolle_ Zsch.) is practically absent. If the bladder be small, the larva is known as a Cysticercoid. For example, the common _Dipylidium caninum_, which lives in the dog, has such a larva, the development of which is explained and illustrated by Figs. 43 and 44. The bladder becomes exceeding capacious in _Coenurus_ and _Echinococcus_. TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND DOMESTIC ANIMALS.[108] I. Scolex in most cases with hooks; uterus with a median and lateral branches; yolk-glands simple, median; genital pore single; dorsal excretory vessel narrower than the ventral, without a circular commissural trunk; eggs without pyriform apparatus (processes of the ovarian membrane) Gen. TAENIA L. (s. str.)