In the Stronolus gigas, a slender nervous ring (a, a, jig. 79) sur rounds the beginning of the gullet, and a single chord is continued from its inferior part and ex tends in a straight line along the middle of the ventral aspect (c, d) to the opposite extremity of the body, where a slight swelling is formed im mediately anterior to the anus, which is surround ed by a loop (e) analo gous to that with which the nervous chord com menced. The abdominal nerve is situated internal to the longitudinal mus cular fibres, and is easily distinguishablefrom them with the naked eye by its whiter colour, and the slender branches (1), b) which it sends off on each side. These transverse twigs are given off at pretty regular intervals of about half a line, and may be traced round to nearly the opposite side of the body. The entire nervous chord in the fe male of this species passes to the left side of the vulva, and does not di vide to give passage to the termination of the vagina, as Cloquet de scribes the corresponding ventral chord to do in the Asraris Lumbricoides. In the latter species, and most other Nematoidea,,a dorsal nervous chord is continued from the cesosphageal ring down the middle line of that aspect of the body corres ponding to the ventral chord on the opposite aspect; but we have not found the dorsal chord in the Strongglus gigas. The nervous system in the latter Entozoon obviously therefore ap proximates to that of the Anellides; but it differs in the absence of the ganglions, which in all the red-blooded worms unite at regular inter vals two lateral nervous columns ; it resembles on the other hand most closely the simple and single ventral chord in the Sipunculus.
Living Ascarides are sensible to different mechanical stimuli applied to the surface of the body, and the sudden and convulsive movements which take place when alcohol, vinegar, or alum-solution are applied to the mouth, would seem to imply that they possess a sense of taste: to light, noise, or odour they are, as might be ex pected from the sphere of their ex istence, totally in sensible.
In those Entozoa which infest the parts of an animal body, where they may be exposed to the influ ence of light, as the gills of fishes, we should not be un prepared to meet with coloured eye specks, or such sim ple forms of the or gan of vision as oc cur in Infusoria and other 'invertebrate animals of a low grade of organiza tion. Nordmann de tected four small round ocelli, of a dark-brown colour, in the Gyroduetylusauriculatus, a Cestoid worm, found in the bronchial mucus of the Bream and Carp ; the eye-specks are situated a little way behind the head, and yield on pressure a blackish pigment. V. Baer observed two small blackish ocelli behind the orifice of the mouth in the Polystomuna Integerrimum, Trematode spe,cies, which infests the urinary or allantoid bladder of the Frog and Toad.
Now this large receptacle is well known to contain almost pure water ; and as the Poly stomum is very closely allied to the Plunarir, which habitually live in fresh water, it is pro bable that the allantoid bladder may be only its occasional and accidental habitation. With respect to the Planarier these are almost univer sally provided with eye-specks, varying in num ber from two, as in the Planuria lactea, (fig,. 80, A) to forty, of a brown or black colour, the external covering of which is tran sparent and corneons. From the experiments of M. Dugi:s* on these non-parasitic Sterelmintha, we learn that when the solar light is directed to the head, they escape from its influence by a sudden move ment, and they also give unequi vocal, though less energetic, proofs of their subjection to the influence of diffused and artificial light. The temporary ocelli observed in the young of certain species of Dis tomat will be presently noticed.
Digestive organs.—We have already alluded to the twu leading modifications of the ali mentary canal, on which the binary division of the Entozoa of Itudo1phi is founded, viz. into SiertImintha or those in which the nu trient tubes, without anal nutlet, are simply excavated in the general parenchyma, and into the Calelmintha, in which an intestinal canal, with proper parietes, floats in a distinct ab dominal cavity, and has a separate outlet for the excrements. In* both these divisions the mouth is variously modified, so as to afford zoological characters for the subordinate groups ; and the alimentary canal itself in the Sterelmintha presents several important differences of structure.
Cystica.—The Cystic worms are generally gifted, as in the species (Cysticercus cellalosa) which occasionally infests the human subject, with an uncinated proboscis for adhering to and irritating, and four suctorious mouths for ab sorbing the fluid secreted by, the adventitious cyst in which they are lodged. In the larger Cysticerci lateral canals may be traced from the suctorious pores extending down the body towards the terminal cyst, but they appear not to terminate in that cavity, the fluid of which is more probably the result of secretion or endosmosis. We cannot, however, partici pate in the opinion of Rudolphi,e that the retracted head derives nutriment from the surrounding fluid of the caudal vesicle, for if that were the case, where would be the neces sity for an armed rostellum in addition to the absorbent pores ? The frequency with which the Cysticerci are found with the head so retracted, may be attributed to the in stinctive action arising from the stimulus of diminished temperature and other changes in the surrounding parts occasioned by the death of the animal in which the hydatid has been developed.