ENTOZOA. 



[ 201 ] 



ENTOZOA. 



cephalic rranfjlion, sondin;i- off branch(>s to 

 the proboscis. In the Treinatoda, of two 

 cesophag-eal ganglia, connected by a trans- 

 verse cord, and sending ofl" two lateral 

 branches, which traverse the body longitu- 

 dinally. In the Nematoidea, it is composed 

 of a single longitndinal cord, furnished at 

 its origin and termination with a ganglion. 



Organs of special sense appear to be 

 absent in the Entozoa, excluding that of 

 touch, which resides in the various cephalic 

 appi'udages. In some, especially in the 

 ciliated embi'yonic form, there are red or 

 black cervical spots, which have been re- 

 garded as eyes ; but they do not appear to 

 contain any refracting body comparable to 

 a lens. Helminthologists have diffi^red as 

 to the presence of a digestive, circulating, 

 and water-vessel system in the Cestoidea 

 and Acanthocephala, certain tubes found in 

 them being regarded as belonging to each 

 of these svstoms bv different authors ; the 



longitudinal vessel-like 

 branches terminatins: in 



tubes with lateral 



a caudal pore, are 

 however, now regarded as excretory organs. 

 In most of the remaining Entozoa, the 

 digestive apparatus is well developed, the 

 mouth distinct, the posterior portion of 

 the alimentary tube much ramified, and 

 terminating either in a cnecal extremity 

 or in a distinct anus. Remarks upon 

 these systems will be found under the 

 genera. 



Propagation. — The Entozoa are propa- 

 gated by spoutaneons division, by gemma- 

 tion or the formation of gemmae, and by 

 sexual organs ; and they illustrate the law 

 of alternation of generations. 



The spontaneous division, which is al- 

 ways ti'ansverse, differs from that of the 

 Infusoria and Zooph\^es, in the new indivi- 

 duals produced not being perfect — a certain 

 numbar of organs only being reproduced, 

 as in the joints (proglottides) of the body 

 of the Cestoidea. 



The formation of gemma? occurs in the 

 larval or cystic forms of Tcenia — Coenurus 

 and Echinococcus. 



In those Entozoa which are propagated 

 by sexes, the individuals are either herma- 

 phrodite or unisexual. In the Cestoidea the 

 sexual organs are usually repeated in each 

 joint, except those near the head. And it 

 appears that there are two Irinds of ovaries 

 — one for the production of the germ (the 

 germinal vesicle and spot), and the other 

 for the yolk. In addition to which, there 

  is mostly a uterus, vagina, testis, penis (spi- 



culum), and vesicula sominalis. TIk." ova 

 are round or oval, often furnislied with a 

 shell, which sometimes has a lid. 



The development of tlie ova of the En- 

 tozoa takes place according to two methods: 

 either the yolk-mass undergoes the ordi- 

 nary process of segmentation, idtimately 

 forming the embryo ; or large transparent 

 embryoiial cells form in the J'olk, the 

 latter not becoming segmented, but under- 

 going subdivision and diminution in size, 

 the growth of the embryonal cells con- 

 tinuing at the expense of the yollf-ma=3 

 until it is entirely consumed ; the entire 

 mass then becomes covered with a delicate, 

 sometimes ciliated, epithelium, and forms the 

 embi'yo. 



In numerous instances, after this primary 

 stage of development — the embryonal-cell 

 condition — has been attained, the embryo 

 does not become directly developed into a 

 form of being resembling the parent ; but 

 the intermediate larval or nurse forms, de- 

 scribed under Ctkxerations, Alternation 

 OF, are produced from it by a non-sexual 

 process ; and ultimately, forming the last 

 stage of the metamorphosis, beings resem- 

 bling the parent, and furnished with sexual 

 organs, are produced. The discovery of 

 the alternation of generations has brought 

 to light the fact that many of the supposed 

 species of Entozoa are only the larval or 

 nurse forms of the true species ; and that 

 many of these forms only complete their 

 stages of metamorphosis when placed under 

 particular circumstances, i. e. in the bodies 

 of different animals, or in different organs 

 of the same animal. 



The foUov.ing arrangement may serve as 

 an index to the articles upon the Entozoa, 

 contained in this work :— 



Order 1. Sterelmintha. Alimentary canal 

 often absent, or not distinct ; wlien pre- 

 sent, with a single orifice only, and 

 branched. 



Earn. 1. Cestoidea (tape-worms). Body 

 strap-shaded,' distinctly or indistinctly 

 divided into transverse joints; male 

 and female organs in each joint ; ali- 

 mentary canal indistinct or none. 



Bnthrioccphalns, Tcenia. 



(Ci/dica) Nm-se or larval forms of 

 Cestoidea : — 



Cysticercus, Coenurus, Echinococcus. 



Earn. 2. Trematoda. Body mostly flat- 

 tened ; alimentary canal distinct, 



u2 



