(360). Carrying out these observations, M. Van Beneden* has not only confirmed the doctrine, but added very materially to our knowledge on this subject, by ascertaining that the TetrarJiynchus undergoes no fewer than four distinct phases of development.

(361). In the first phasis of its existence, the worm is more or less vesicular in structure, being armed with four suckers, and a sort of proboscis in the centre. It is possessed of extraordinary contractility; and in different species there are spots of black pigment, representing eyes. In this condition these worms have received from helminthologists the name of Scolex (Scolex polymorphus; Scolex Acalepharum), Sars, Tetra-stoma Playfairii, Eorbes and Goodsir, Dithyridium Lacertae, etc. These are more especially found in the pyloric caeca.

(362). The second phasis is, perhaps, the most curious. In the interiorof the Scolete there is formed a Tetrarhynchus, by a process of gemmiparous reproduction; and from the surface of the latter a kind of viscid secretion exudes, which becomes solid, and forms a sort of sheath made up of concentric layers.

* Ann. des Sc. Nat. 3 ser. x. p. 15.

(363). In this state of development there is found a sheath formed of several layers, in the interior of which is a Trematode worm (Amphi-stoma rhopaloides, Ch. Le Blond); and in the interior of the latter may be perceived a Tetrarhynchus, which moves about vivaciously as soon as its prison is opened. This Tetrarhynchus has been regarded by naturalists as a parasite inhabiting the Trematode worm; M. Van Beneden believes it to be a moveable gemma (bourgeon mobile).

(364). It is constantly, if not always, found in cysts formed at the expense of the peritoneum, in a great number of sea-fish - cod, trigla, conger, etc. In the third state of its existence, the Tetrarhynchus is free, but in all respects resembling that which was enclosed in the Trematode worm; in a short time, however, transverse lines become developed upon the posterior part of its body, segments are formed, and it becomes Tcenioid. In this condition it has been named Bothrioce-phalus, or more recently Bhynchobothrius. It is found in the intestinal canal of the skate, among the first turns of the spiral valve.

(365). In the fourth and last phase of its growth, it presents a more simple structure, the perfect animal performing the part of a tube destined to disseminate ova. In this condition it is nothing more than the last segment of the Taenioid form detached, in fact a Proglottis. In this condition it is found in the intestine of the skate, in company with the Bothriocephali: this is the mature or adult animal, provided with complete male and female sexual organs.

(366). The adult Entozoon (the Proglottis loaded with eggs) is evacuated together with the fasces of the skate, and with its ova serves as food to fishes of small dimensions. The ova are developed either in the intestines or the intestinal caeca of the devourer, and if the fish which contains them happens to be swallowed by another fish, the development still proceeds in its alimentary canal, or the caeca thereunto appended. When arrived at the condition of a complete Scolex, after having perhaps passed through the stomachs of several fishes that have successively devoured each other, it perforates the intestinal walls and lodges itself in the peritoneum, in which situation it forms its sheath, and produces in its interior the "moveable gemma," from which is produced a Tetrarhynchus. The fishes containing the latter form are swallowed in turn by the voracious Rays and Sharks, and their flesh having been dissolved in the stomachs of their devourers, the Tetrarhynchus becomes free, and continues its growth in the intestines until the last forms (Proglottis) are complete, which alone are furnished with a sexual apparatus.

Thus, from the production of the egg to the completion of the mature animal, these parasites are continually passing into the alimentary canals of new fishes; and it is only under such circumstances that they seem to attain their full development.

(367). Cystoidea. Transformation Of Cystiform Entozoa Into Taeniae

The gradual transformation of Cysticercus piriformis into the Tamia serrata has been established by feeding young dogs with the cystic parasites still enclosed in the cysts in which they are found in the omentum of rabbits*. The first effect produced upon the Entozoa thus enclosed in their cysts, after they have been swallowed, is the solution of the cysts by the gastric juice in the dog's stomach, after which the caudal vesicle of the Cysticercus pisiformis is attacked and destroyed by the same digestive agent, leaving nothing of it remaining but the whitish and rounded Scolex, which, passing through the pylorus, becomes attached to the walls of the duodenum, in which situation it has to await its subsequent growth. At the posterior end of the now tailless cyst-worm, the point at which the caudal vesicle was previously attached is distinctly indicated by a sort of cicatrix. Subsequently the growth of the Entozoon commences, its transverse wrinkles are multiplied, and in the course of a few days the body becomes divided into segments, which, at first very short, elongate and soon present the marginal generative pores.

After a residence of twenty-five days in the intestines of the dog, the Taenia has attained the length of from 10 to 12 inches, and in three months 20 or 30 inches or more, at which time the posterior joints appear to be sexually matured, and the last segments (Proglottides) become detached. The ova enclosed in the ripe joints are seen to be completely developed, and contain in their interior the mobile embryo armed with its six hooklets.

It must now be an important task for helminthologists to trace the further development of the embryos produced from these eggs, in order to determine the mode of origin of the Cysticercus piriformis.