II. TAPEWORMS. CESTODA.--This comprises not only the tapeworms, but also the measles and other bladder-worms or cystic Entozoa of the old authors (Cystica). The Greek word _kestos_ means a band or girdle; hence the ordinal term above given. The bladder-worms, including Hydatids, Cysticerci, &c., are the larval stages of growth of various tapeworms. The further reduction of this order into sub-orders or families requires careful attention. At present we have _Tæniadæ_, _Acanthotæniadæ_, _Dibothridæ (= Bothriocephalidæ)_, _Diphyllobothridæ_, _Tetrarhynchidæ_, and _Tetraphyllobothridæ_. All the genera and species are entozoal. The proposal to separate the snouted or proboscidiform tapeworms (_Rhynchotæniadæ_) from those in which the rostellum is absent (_Arhynchotæniadæ_) does not recommend itself to my judgment. III. ROUNDWORMS. NEMATODA.--This series comprises not only lumbricoid or roundworms proper, but also threadworms. The term derives its origin from the Greek word _nema_, signifying a thread. It likewise includes the strongyles, the term _strongulos_ meaning round or cylindrical. This is a very extensive group whose parasitic members are strictly entozoal, whilst the non-parasitic forms are either entirely free or they infest plants. Some of the so-called free nematoids live in the slime of animals. The artificial classification by Schneider, based on the muscular system, places these parasites in three well-marked groups, but I think it a disadvantage to separate widely many really closely allied forms. Thus, in his _Polymyarii_ we have the genus Enstrongylus, and in his _Meromyarii_ the Strongyli proper. Most of the genera may be fairly included in the following families:--_Ascaridæ_, _Cheiracanthidæ_, _Cucullanidæ_, _Strongylidæ_, _Trichinidæ_, _Oxyuridæ_, _Trichocephalidæ_, _Filaridæ_, _Gordiidæ_, _Anguillulidæ_. IV. THORNHEADED-WORMS. ACANTHOCEPHALA.--This group embraces a small series of parasites, which, in general appearance, resemble the nematode worms. They differ, however, essentially, being, as the term indicates, furnished with spine-covered heads. They are, moreover, destitute of digestive organs. The species are entozoal in habit, abounding particularly in fishes and reptiles. At present, all the known forms are included in one family (_Echinorhynchidæ_), which also comprises only a single genus.