TACHIDIUS. 



[ 750 ] 



Five fossil species. 



BIBL. Kiitz. Sp. Alg. 118; Smith, Br. 

 JDiat. ii. 44; Pritchard, In/us. 807. 



TACHID'IUS, LiUj. A genus of Cope- 

 podous Entomostraca. T. brevicornis ; ma- 

 rine. (Brady, Copep. ii. 19.) 



TACHYGO'NIUM, Nag. A doubtful 

 genus of Palmellaceee. (Rabenh. Alg. iii. 

 36.) 



TADPOLE. See FROG. 



TJ2NIA (Tape-worm). A genus of Ces- 

 toid Entozoa. 



Char. Body elongate, compressed, jointed. 

 Head mostly broader than the very narrow 

 neck, with 'four suctorial depressions ; and 

 usually a median, imperforate retractile 

 rostelluni, very frequently armed with one 

 or two circles of minute recurved hooks, 

 especially in the young state. Genital ori- 

 fices situated at the margins of the joints or 

 proglottides, either on one side only, or on 

 both margins and on alternate joints. 



The TcenicBj of which the common tape- 

 worm may be taken as the type, are found 

 in vertebrate animals alone, and in these 

 only in the alimentary canal. They are 

 most common in birds, next in mammalia, 

 then in fishes, and lastly in reptiles. 



The species are very numerous : Rudolphi 

 enumerated 146 ; Dujardin 135. 



Tcenia solium,ihQ common human English 

 species, varies in breadth from 1-50 to 1-40" 

 at the anterior part, to about 1-3" at the 

 middle and posterior part. At the anterior 

 extremity is situated a central rostelluni, 

 which is surrounded by a crown of small 

 recurved hooks, as in PL 21. figs. 1 / & 10. 

 Behind these are four suctorial depressions, 

 which are not pervious at the bottom. The 

 digestive system is ^wanting, and the worm 

 absorbs by its surface; but according to 

 Blanchard, it is represented by two tubes or 

 lateral canals (PI. 21. fig. 14 a), having be- 

 tween them a transverse canal at the sum- 

 mit of each joint; these extend from the 

 anterior to the posterior end of the body. 

 In the cephalic portion, directly behind the 

 suckers, there is a kind of lacuna or furrow 

 communicating directly with these intesti- 

 nal tubes ; and it appears that the nutritive 

 matters respired by means of the suckers 

 penetrate into this lacuna, and thence into 

 the digestive canals. These tubes have di- 

 stinct walls, and are best seen when the animal 

 has been macerated in water, and is exami- 

 ned by transmitted light, or after having 

 been injected. But this is very doubtful. 



The 'vascular system, according to the 



above author, consists of four longitudinal 

 vessels (PI. 21. fig. 14 V) situated a little 

 above the intestinal tubes, and infinitely 

 more slender. They traverse the whole 

 length of the body; and between them 

 are numerous transverse vessels (PI. 21. 

 fig. 14). These so-called digestive and vas- 

 cular canals are now considered as belonging 

 to a water-vessel system ; the canals unite 

 in the last segment in a receptacle. 



The male generative organ consists of a 

 slender coiled tube, extending to near the 

 principal ovigerous canal, where it is pre- 

 ceded by some very small testicular capsules 

 (PI. 21. fig. 14 c). The slender tube termi- 

 nates in a duct (fig. 14 d), which opens into 

 the lateral orifice, or sometimes it projects 

 externally in the form of a spiculum. The 

 ovary consists of a principal median canal, 

 presenting slight flexuosities, and extending 

 nearly from one end to the other of each 

 joint. It presents C86cal branches on both 

 sides, and opens by a slender oviduct (fig. 

 14 e) just within the genital orifice. 



The ova are innumerable ; one is figured 

 in PI. 21. fig. 15. They consist of an outer 

 delicate membrane, enclosing a gelatinous 

 substance containing numerous highly re- 

 fractive globules. Within this is another 

 very delicate and transparent membrane, 

 closely applied upon a brittle, dark-looking 

 (by transmitted light, but white by reflected 

 light), thick envelope, within which is the 

 yolk or embryo, according to the state of 

 development of the ovum. Very frequently 

 the hooks of the young tsenia are seen im- 

 bedded in its centre, as shown in the figure. 

 The thick brittle coat of the ovum exhibits 

 an appearance of radiating fibres (canals ?) ; 

 and when broken, the fractures are radiant. 

 When the middle of the outer surface of 

 the brittle envelope is brought into focus, it 

 presents a tolerably regular appearance, as 

 if composed of cells; this arises, however, 

 from the extremities of the fibres being 

 brought into focus. 



The filiform spermatozoa are readily found, 

 simply by picking any joint containing ova 

 to pieces with needles. 



The young animal, consisting of head and 

 neck only, was formerly considered distinct, 

 and placed in a genus (Scolex). 



Among other species may be mentioned : 

 Tccnia mediocanellata, the unarmed tape- 

 worm of the ox ; T. crassicollis, in the cat ; 

 T. echinococcus, in the dog and wolf ; T. 

 serra'a, in the dog ; and T. nana, human, in 

 Egypt. 



