$103. THE HELMINTIIE3. 107 



The young of EcMnococcns, Coenurus, Cysticercus, and many of the 

 Taenia, have their head armed with a circle of single or double hooks, 

 which were known to the oldest Helniintholoc;ists. Each hook consists of 

 a strongly-curved point, situated upon a round, straight pedicle, of varia- 

 ble length. At the point where the curve ceases, there is, upon the con- 

 cave side of the organ, a small conical process. When this circle of hooks 

 is unfolded, the points project around the anterior part of the head, whilst 

 the pedicles point towards the inner and their processes towards the poste- 

 rior portion of the body, and are buried in the parenchyma. Both arc 

 surrounded by muscular substance. When, therefore, the muscles of the 

 pedicles contract, the hooks are drawn downwards and outwards, and their 

 points are brought together upon their convex surface in the long axis of 

 the head ; but when, on the other hand, the muscles of the processes con- 

 tract, these last are depressed, the pedicles are again elevated, and the 

 hooks project outward. With many Cestodes, this circle of hooks is situ- 

 ated upon a particular proboscis (rostellum), which can be retracted into a 

 sheath which is concealed between the four suckers of the head.'^' 



With Aathocephalus, Gymnorhjnchus, and Tetrarhynchus, there are 

 upon the head four long and completely retractile probosces, which arc 

 armed with an extraordinary number of small, backwardly-curved hooks 

 which are attached by a large base to the external surface of the organ, 

 and are without special muscles ; by these, these animals can penetrate the 

 most compact animal tissues. Each proboscis is a hollow muscular tube, 

 which can be voluntarily retracted within a sheath of the same nature, and 

 then the hooks, with their points directed in front, are drawn together in 

 its axis. The length of the sheath, which is usually enlarged at its base, 

 depends upon that of the proboscis. In many species of Tetrarhynchus, 

 they reach far into the neck of the animal.*^' 



The Acanthocephali have only one of these organs, and the hooks, which 

 are without special muscles, form rows arranged one after another. Both 

 the number of these rows and the form of the hooks vary in different 

 species. Usually their size decreases from before backwards, so that those 



the suctorial apparatus of the Trematodes. laTris- of which iViYzscA {Ersch and Grufter's Encyclo- 



tomum hamatum (see Rathke, Nov. Act. Acad. paid. VI. p. 49, IX. Taf. 11. A.) has made a sep- 



Leop. Cai'jl. XX. 18i3, p. 241, Taf XII. fig. 11), arate genus ; the females have a protrusive sting 



several sharp points project from the bottom of the in the sucker situated at the posterior eirtremity. 



suclier at the posterior extremity. With Polysto- 4 With Echiiiococcus, Coenurus, and Cysti- 



mvm appendiculatum (Nordmann, Micrograph, cercus. the number of hooks is twenty to thirty ; 



Beitrige, Hft. I. p. 82, Taf. V. fig. 6, 7), the borders and I have seen as many with Taenia scolecina, 



of the six suckers at the posterior extremity are and infundibuliformis ; but I have found only 



armed with a sharp claw. The disc of Gyrodac- eighteen with Taenia an^ulata, ten with Taenia 



tylus (Il)id. Taf. X.) has its borders provided with seti^era, and eight with Taenia lanceolata. 



six horny points, and its base is supported by two Taenia scolecina, crassicollis, and Cysticer- 



sides of the same nature, curved like an arc. cus, have an equal number of large and small 



A very complicated support, formed of horny hooks alternating with each other, and, at a cer- 



arches and ridges, sustains the eight suckers at the tain point, farming a double circle, 



posterior extremity of Octobothrium sasittatum, With Taenia, and especially those which have 



Merlangi, and of Diplczoon paradoxum ; an these organs on the proboscis, they may be partly 



analogous support wholly surrounds the large foot or even wholly detached. 



at the end of the body of Axine {Leuckart, Zool. Rudolphi has regarded Taenia gracilis, angu- 



Bruchstiicke lift. 3, Taf. II. and Nordmann, lata,infundibuli/ormis,seti^era,3.ndstylosa,as 



Micogr. Beitr. lift. 1, Taf. A'll.; also Diesing; naturally without these organs, but I have often 



Nov. Act. Acad. Leop. Carol. XVIII. pt. 1, Tab. found them having a complete circle. 



XVn.). With Taenia cucumerina, the structure is dif- 



The four fossae found on each side of the mouth ferent ; its seven rows of hooks are in all respects 



of Pentastomum contain simple and double very hke those oi Echinorkynchus. 



curved hooks, which the animal can erect at will 5 See Leblond, Ann. d. Sc. Nat. VI. 1S30, PI. 



(Diesing, Ann. d. Wiener Mus. I. Abth. 1, Taf. XVI. fig. 5, 6, 7 ; and Goodsir, Froriep^s neue 



III. IV.). A remarkable exception among the Notiz. 1841, No. 429, fig. 18 ; also Mayer, Mul- 



Nematodes is found with Hedruris androphora, ler^s Ai'ch. 1842, Taf. X. 



