HYDRA. 



[ 399 ] 



HYDRACHNA. 



the form of the parent, while remaining 

 attached to it. Sometimes several of these 

 are formed upon a single individual at the 

 same time, and so, remaining adherent, 

 thev give the animal a branched appearance 

 (PL 41. fig. 21). 



At certain seasons of the year, as at the 

 end of summer or in the autumn, reproduc- 

 tion takes place by the formation of sperma- 

 tozoa and ova. The spermatozoa are formed 

 within spermatic capsules. These arise as 

 minute conical tubercles a little beneath the 

 base of the tentacles, one on each side 

 (PI. 41. fig. 24 a) ; and the spermatozoa are 

 liberated from them by bursting. The sper- 

 matozoa resemble those of the Mammalia, 

 except that the tails are undulate. The ova 

 are furnished with a thick coat, and are 

 formed in the substance of the lower part 

 of the body (fig. 24 6). They subsequently 

 separate from the body, and appear to be 

 capable of spontaneous motion; but whether 

 from the presence of cilia or not, is unde- 

 cided. The ovisac then becomes ruptured, 

 and the new animal escapes (fig. 25). 



Hydrce are very common. The best me- 

 thod of procuring them is to collect a number 

 of water-plants from any clear pool or slow 

 stream, and bring them home in an india- 

 rubber bag (sponge-bag). On placing the 

 Elants subsequently in a glass jar (con- 

 3cti oner's jar) containing water, they will 

 be found at the end of some hours with the 

 tentacles fully extended in search of prey, 

 when they are easily recognized. They 

 usually adhere to the sides of the glass, or 

 to the stems or undersides of the leaves of 

 the plants ; but sometimes they are seen 

 suspended from the surface of the water by 

 the sucker, which is protruded just above 

 it so as to become partly dry. A number 

 of small Entomostraca should be added to 

 the water, as the Hydros are very voracious. 

 Some of the species of Hydra are occa- 

 sionally covered with minute parasitic Infu- 

 soria, viz, K&'ona polyporum (PI. 50. fig. 13), 

 which is found upon H. mdqaris and fusca, 

 and Trichodina pediculus (PI. 31. fig. 16), 

 which occurs upon H. wdgaris and viridis. 

 It is an interesting sight to see these running 

 up and down the tentacles and surface of the 

 body of the polypes, when we recollect that 

 their surface is covered with the stinging 

 organs, These lice are not, however, found 

 in any numbers upon perfectly healthy 

 polypes, impurity of the water and an un- 

 healthy state being generally denoted by 

 their presence. 



Parker recommends osmic acid in the 

 examination of the minute structure of 

 Hydra-, and hardening in bichromate of 

 ammonia for sections. 



BIBL. Leeuwenhoeck, Phil Tr. 1703, 

 xxiii. ; Trembley, Polyp, d'eau douce ; Lau- 

 rent, Rech. s. VHydre ; Corda, Ann. Sc. 

 Nat. 2 ser. viii. ; Schaefler, D. Armpolyp. ; 

 Erdl, Mutter's Archiv, 1841 ; Ecker, Sieb. 

 and Koll. Zeitsch. i. ; Johnston, Br. Zooph. ; 

 Thomson, Todays Cycl. An. and Phys. iv. 

 17 ; Hancock, Ann. N. H. 1850, v. 281 ; 

 Allnian, M-icr. Journ. 1854 ; Hincks, Br. 

 Zooph. 309; Kleinenberg, Hydra, 1872; 

 Allman, Qu. M. Jn. 1874, xiv. 1; T. Parker, 

 Pr. Hoy. Soc. 1880, 61. 



HYDRACH'NA, MiiU. A genus of 

 Acarina, family Hydrachnea. 



Char. Palpi tolerably long, third joint 

 longest, the fourth and fifth terminated each 

 by a claw ; mandibles ensif orin ; rostrum 

 long, scarcely shorter than the palpi ; body 

 rounded ; eyes distant ; vulva concealed by 

 a plate or shield. 



When young, these little water-spiders 

 have three legs only, and in this state have 

 formed another genus, Achlysia. Several 

 species : 



H. crttenta, Mull. = H. globula, Herm. 

 (PI. 6. fig. 29). Body subovate ; two pairs 

 of eyes at a moderate distance apart, reni- 

 form, dark red ; skin covered with minute 

 puncta. 



The rostrum is broad and curved at the 

 base (fig. 29 c, the lower part directed to the 

 left), cleft above, so as to form a kind of 

 channelled sheath, containing the anterior 

 narrower portions of the two mandibles (b). 

 The palpi (c, upper organ) are inserted upon 

 the sides of the base of the rostrum and 

 curved downwards ; the first joint is very 

 broad, the second much curved, the third 

 long, and flattened on one side and rounded 

 on the other ; the fourth joint is short, and 

 terminated by a short and thick claw ; the 

 fifth also forms a claw, but the two claws do 

 not form a chela, their curves being parallel. 

 Of the legs (fig. 29 a), the three posterior 

 pairs are ciliated for swimming, and the 

 posterior are much longer than the anterior ; 

 the coxae are flattened and form two groups 

 on each side ; between the two posterior 

 coxae is the orifice of the reproductive 

 organs ; the tarsi all have two claws, and 

 are obliquely truncated and concave at the 

 end (fig. 29 e). 



The eggs are reddish-brown and deposited 

 upon the stems of water-plants ; the nymphse 



