1. The reservoir of Haswell (vesicula resorbiens of Merton) is an important part of the female reproductive apparatus of Caridinicola and Monodiscus. It lies between the oviduct and the germiduct, and its rôle in these two genera differs from that in Temnocephala. Histologically it is alike in the three genera.
2. In Caridinicola the reservoir stores spermatozoa received in copulation; it receives yolk from the vitelline glands and also the egg from the germarium.
3. In Monodiscus the reservoir accommodates the egg from the germarium as in Caridinicola, but the spermatozoa are stored in a single, comparatively large receptaculum seminis found ventral to the reservoir.
4. Surplus reproductive material in Caridinicola passes out through a temporary reservoir canal, to be absorbed by the intestine. In Monodiscus, where the vitelline duct opens into the genital atrium, there is less surplus material in the reservoir, so that the reservoir conal is vestigial.
5. The receptaculum seminis is absent in Caridinicola, but is present in Monodiscus as a single vesicle, which opens into the reservoir and not into the oviduct, as in Temnocephala.
6. Comparative study shows that the reservoir is primitively a part of the oviduct (like the ootype), enlarged in this case to accommodate spermatozoa and yolk to supply the egg received from the germarium. This condition is found in Caridinicola. In Monodiscus a receptaculum seminis is cut off to contain the spermatozoa. Finally, in Temnocephala the reservoir is a vesicle which appears to be distinct from the oviduct, and the receptacula seminis open separately into the oviduct.
7. The reproductive apparatus is most primitive in Caridinicola; while Monodiscus shows an intermediate stage between that of Caridinicola and that of Temnocephala.