OVUM. 



11 



so called, are now known to be capable of 

 true sexual generation, by means of ova, in 

 their perfect or complete condition, and the 

 whole class is remarkable for the great de- 

 velopment of the sexual organs, and the pro- 

 digious numbers of ova which they bring forth. 

 But it has been ascertained that their ova are 

 rarely developed into new beings in the place 

 of the abode of the adult entozoa : they are 

 commonly subject, therefore, to migration 

 from one organ to another in the same indi- 

 vidual, or from one animal to another, or 

 from the parasitic to the free-living condition ; 

 and they have recently been discovered to 

 present very remarkable changes of external 

 form and internal organisation in their va- 

 rious habitations ; so great, indeed, that many 

 of them, previously believed to belong to 

 species, and even to genera and families 

 widely different, are now recognised as dif- 

 ferent conditions of the same animal or 

 species, and that many forms, whose mode 

 of generation was unknown, are found to be 

 derived by indirect production from ova, in 

 a manner which will be more particularly de- 

 scribed under the next section. 



Thus it appears that the only entozoa 

 which are destitute of sexual organs, viz. 

 those belonging to the division cystica, are 

 very probably only imperfect forms of Taenia 

 or other cestoids, which, so long as they are in 

 the encysted or confined condition, do not 

 reach their full development : but many of 

 which, during their incomplete condition, are 

 capable of being multiplied by a process analo- 

 gous to gemmation. 



The greater number of the entozoa breed 

 only when in the alimentary canal of animals, 

 and the ova are excreted along with the 

 foeces : it is obvious, therefore, that very 

 many ova must be destroyed, and that a few 

 only are liable to gain those peculiar situa- 

 tions which are fitted to maintain them in 

 their earlier conditions, or in their later stages, 

 to bring them, as parasites, to their full state 

 of development. 



The entozoa are usually found, therefore, 

 in their most advanced stage, in the alimentary 

 canal. There seems, on the whole, little dif- 

 ficulty in accounting for the entrance of en- 

 tozoa from without into the alimentary canal, 

 or the pulmonary air-cells and other open 

 cavities : and every new fact that has been 

 observed relative to the occurrence of entozoa 

 in man and animals, leads to the conclusion 

 that the ova, or perhaps more frequently the 

 earlier larval or undeveloped forms of the 

 entozoa, gain access to these situations by 

 introduction from without, and most fre- 

 quently along with food and drink ; in those 

 instances at least in which the entozoa migrate 

 from one animal to another, or from an 

 animal to the free state before returning 

 to the parasitic condition. But the entozoa, 

 which are, in general, in an incomplete state 

 when situated in the close cavities or solid 

 textures of the organs of animals, sometimes 

 make their way from these situations into the 

 alimentary canal, there to undergo their finul 



development. Surh appears to be the case 

 with the Strongylus urmatus, living in an 

 incomplete state in aneurismal sacs of the 

 blood-vessels of the horse, and in a fully 

 developed state in the intestine ; the Stron- 

 gylus vagans, in cysts of the porpoise, and 

 afterwards free in the lungs ; the Ligula or 

 Bothriocephalus solidus, in cysts of the ab- 

 dominal cavity of fishes, and afterwards in 

 their perfect state in the alimentary canal of 

 sea-birds. The Trichina and Echinorrhynchi, 

 imbedded in the muscular flesh in great quan- 

 tities, are no doubt imperfect forms of other 

 worms, which must migrate from these situa- 

 tions to attain to their complete state. 



With regard to the manner in which the en- 

 tozoa inhabiting the close cavities of the body, 

 or imbedded in the solid substance of organs, 

 either in the free or encysted condition, gain 

 access to these situations, which has to many 

 appeared inexplicable, excepting on the hypo- 

 thesis of their arising actually in the places 

 which they inhabit, observations are no less 

 decided in proving them to be of external in- 

 troduction. 



In the first place it may be stated that, al- 

 though the ova of a considerable number of 

 the entozoa are of so considerable a size as to 

 render it improbable that they have passed as 

 such through the capillary vessels, yet few, if 

 any, of these larger kinds are observed en- 

 cysted, and in others the ova are extremely 

 minute, and might, without difficulty, be car- 

 ried through most of the capillary vessels. 



In the next place it may be mentioned that 

 the embryoes, or earlier forms of various 

 parasites, and the ova of others, have been 

 observed in considerable numbers in the cir- 

 culating blood of various animals *, as showing 

 that by this means the entozoa may be carried 

 in their small and early condition into any 

 part of the body of an animal which is fitted 

 to afford the conditions favourable to their 

 farther development. 



But in what manner have these bodies 

 gained an entrance into the blood-vessels, or, 

 in other instances, how may entozoa have 

 penetrated into cavities or the parenchjma of 

 organs, without being conveyed through the 

 blood-vessels ? To this question, also, recent 

 observations seem to furnish a satisfactory 

 answer : for it has been ascertained that, in 

 a number of instances, smaller or larger en- 

 tozoa, but especially the former, pierce the 

 tissues of animals with great apparent facility, 

 being frequently provided in the young state 

 with an apparatus of sharp hooks for that 

 special purpose. Some of them have been 

 observed in the act of passing through the 



* I may here refer to the original observations of 

 Schmitz, (Berlin, 182G), and the more recent ones of 

 Valentin Gruby, Gluge, Vogt, and others. See 

 Valentin, Kepertorium for 1842 and 18-13. The 

 Annual Kepoit in Muller's Archiv. for the same 

 years, and in Wiegmann's Archiv. for Naturgcsrh. 

 Valentin's account of the < )va of Distoma in the fluid 

 covering the medulla oblongata of a foetal sheep 

 (Miil!er's Archiv. 1840, p. 317), and V. Siebuld'.s 

 Article 'Parasites' in II. Wagner's Handworterbuch 

 cler Physiologic. 



