G44 EXTOZOA. 



upon -which they live, when their numbers become excessive, are well known ; and we 

 may mention, that the best remedy for those infesting the human intestines, appears 

 to be animal oil mixed witli spirits of tur})entine. 



The class admits of division mto two orders, of which the organization is so dif- 

 ferent that they might, perhaps, be considered as two distinct classes ; or, at all events, 

 subclasses. These are, — Entozoa NEMATOiDiCA, or Cavitied Entozoa, which have the 

 intestine floatimi; in a distinct abdominal cavity, and commencing at a mouth and ter- 

 minating at a vent; and Entozoa Parexchymata, which have the viscera obscure, 

 generally in the form of vascular rumihcutioas, and sometimes not at all discernible. 



THE FIRST ORDER OF THE ENTOZOA. 



NEMATOIDKl. 



The members of this order have an external skin, more or less provided with muscular 

 fibres, and striatc-d transvL^-sclv. They havu an intestinal canal running distnictly tlirougli 

 thu whole length, and attached to the skui or tunic of the body by many filaments, whicli 

 appear to transmit nourishment. There is no circulation ; but, in some species, there are 

 two cords cxtendmg from a ring roniid the mouth, which are understood to be nerves. Re- 

 productive organs are apparent in all, and in some they are 

 greatly developed; nor is the reproductive energy of the 

 animals under circumstances favourable to its develupcrnent 

 Fig. 140.— Pentaitoma tffiiioidc3. Icss activc than the organs would indicate. This order forms 



only one family, but contams several genera. 



FiLARiA (Tliread-worra), — 

 Has the hodv long, slfinler, and thread-lilce, resembling that of the GorrJii among Aruielida?, but 

 with mere marks on the body instead of the rings. Tlio nioutli is a cireular opening at the anterior 

 exlremity. They are not fuund in the open cavities, Init are imbethled in the parenchyma of the 

 celbilar tissues, Ijctween the coats of the viscera, and in other situations: they often exist in numerous 

 bundles, contained in a commou cyst or tunic. They are nut confined to the larger animals, but are 

 found in insects and their larva', and even in various Mollusca. 



The most common, or at all events the most dreaded by i\lan, is the Guinea Worm, F. Mcdinensu. This trou- 

 blesome animal is very common in hot climates, ^vhcre it insinuates itself under the skin, [generally of the leg:, and 

 is said to gnaw to the ]en,c;th of ten feet, or more. Accordino; to the accounts it will, if undisturbed, remain in the 

 body for a long time without causing much uneasiness ; but, if it is disturbed, it is said to cause the most excru- 

 ciating pain, especially if it finds its way to a very sensitive part of the body. When it shows itself externally, it is 

 extracted very slowly for fear of breakmg it, as, if that takes place, its position in the body retreats more inwardly, 

 and causes great agony and convulsions. It is about the size of the tube of a Pigeon's quill, and has the tail ter- 

 minated by a sharp trunk. The sexi;s are in separate animals, but the mode of propagation is a little obscure. 



Trig HOCEPH ALUS, — 



Have the body round, thread-like in the aiiterlur })art, and terminating in a round month; and the 

 posterior part of the body is considerably thicker. 



T. dispar, is the most common species. It is from an inch to two inches in length, and thick for about the last 

 third. The thick part of the male is spirally convoluted, and the organ of generation is conspicuons. In [he 

 female it is more straight, and has a simple opening. It is one of the worms of most fiequent occurrence in the 

 human intestines ; and, in some diseases, it multiplies very rapidly. 



TricAoA'^oma, have the anterior part of the body tapering gradually to the mouth; and Ori/unsy has the tall 

 slender and thread-like. One species of the latter, O. curvata, from an inch to three inches long, is found in the 

 co£cum of the Horse. 



Cuciillanii-M, has the body cylindrical, but thinner in the antrrinr jmrtion. Tlie head is blunt, and enveloped in 

 a '■ort of hood. This gei]Li>. has hitherto befn found in the iiitustines >}{ Fi.shes only. < )ne species, C. hinislrin, is 

 common in the Perch, the Pike, and other Fishes. It is about an incli long, about the thickness of a tln-ead, and 



