896 



THE ARACHNOIDAE. 



§320. 



rudimentary penis. It is yet undetermined if the two external, lateral, 

 pectinifbrm appendages situated near the genital orifice in both sexes, 

 serve any purpose during copulation. ^^"^ 



§ 320. 



"We have, as yet, only insufficient observations on the Development of the 

 Arachnoidae. However, those that we possess upon its first stages,^'* show 

 that, with few exceptions,'-' here as with the Crustacea, the disappearance 

 of the germinative vesicle is followed by a superficial and partial segment- 

 ation of the vitellus. There is thereby formed a thin embryonic layer, com- 

 posed of molecular corpuscles retained in a transparent viscid liquid, and 

 distinguished from the rest of the vitellus by its white color. While this 

 oval blastoderma, whose longitudinal axis corresponds to the ventral or 

 nervous side of the ^mbryo, extends towards the sides and the back, it 

 divides into an external or serous, and an internal or mucous lamella, 

 the last of which gradually covers the i-emaining portion of the vitellus, 

 and becomes changed into the digestive tube and its appendages. In 

 the mean while, there appear, on the external surface of the serous lamella, 

 Tarious symmetrical prominences and projections, which in time become 

 the segments of the body, the parts of the mouth, the tactile, and the 

 locomotive organs. 



With the exception of the Scorpionidae, and Oribatea, which are vivi- 

 parous, the embryos of all the Arachnoidae are developed subsequent to the 

 deposition of the eggs.*''' 



With the majority of Arachnoidae, the embryos, at their escape from the 

 ^gg, have the form of the adult.*** The lower orders, only, form an 

 exception in this respect, for they acquire their definite form after several 

 moultings, and a true metamorphosis. With the Pycnogonidae, these 



V)Tulk (Ann. of Nat. Hist. XV. p. 56) has 

 btely expressed the opniion that these combs serve 

 to clean the pal])i, the tarsi, and the extremity of 

 the tail. He adduces, as proof, the presence of 

 transparent combs of exactly identical form, with 

 Ohisium, I>et\veen the jiincers of the cheUceres, and 

 which are used fcjr this purpose. 



1 The first phases of development have not 

 been observed as yet except with the Araneae and 

 Scorpionidae ; see Herold, De Generat. Aranear. 

 in ovo, 1824 ; Rnthki, Zur. Morphol. Reisebemerk. 

 aus Taurien, 1837, p. 17, and in Burdach''s 

 Physiol. II. 18.37, p. 242 ; the same in Froriep^s 

 neue Notiz. XXIV. 1842, p. 165 {Lycona nac- 

 eata) ; also KüUiker, in Muller^s Arch. 1843, p. 

 139 (Scorpio europaeus) ; finally, fVittich, Observ. 

 quaed. de Aranear. ex ovo evolut., Halis. 1S45, fig. 

 1, A. As to the development of the Acarina, I 

 have satisfied myself upou the eggs of the Oribatea, 



that the same phenomena occur as with most of the 

 other Arachnoidae.* 



2 I have distinctly seen, in the eggs of Macrohi- 

 otus Hufelandii. the segmentation involve the en- 

 tire vitelius. KoJIikrr {]\Iullrr\t Arch. 1843, p. 136) 

 has made: the same nlisrrvaticm with Fijr.no^oninn.\ 



3 The relations «i tlie Sriirpiiiniilae in this respect 

 are very remarkable, for their embryos are devel 

 oped in the ovaries at the spot even where tlve eggs 

 are formed ; see MiUte.r, loc. cit. p. 55, and Rathki^ 

 Zur Morphol. loc. cit. It is evident that the siicrm 

 must ascend from the two seminal reservoirs into 

 the ovaries to fecundate the eggs. With the Ori- 

 batea, the embryos appear to be developed in a 

 kind of uterus situated immediately behind the 

 oviiiositor. 



4 With the Araneae, the sexual differences which 

 are so striking do not appear until after the first 

 moulting. 



* [ § 320, note 1.] See, also, JVilnon, Researches 

 into the structure and develop, of a newly-discov- 

 ered parasitic Animalcule, &c., in the Philos. Trans. 

 1844, p. 305 (Entozoon {Demodex) folliculorum), 

 and Van Heneden, Recherches sur I'Atax upsilo- 

 phora, in the M<jm. de I'Acad. Roy. de Bruxell. 

 XXIV. fVilsori'.'i details are imperfect, and throw 

 but little light on the real character of the develop- 

 ment of the follicle-jjarasite. It would appear, 

 however, to be truly one of the Arthropoda. — Ed. 



t [ § 320, note 2.] i'ur the embryology tjf the 



Tardigrada, see Kaufmann, loc. cit. in Siebold 

 and KöUiker's Zeitsch. III. 1851, p. 220. The 

 type of development is like that of the Articulata in 

 general, and this would seem to clearly settle the 

 position of these animals in this class. Kauf- 

 mann confirms the observation of Siebold as to 

 the segmentation of the entire vitellus. After this 

 process has occurred, the mulberry like mass is 

 3hanged into the embryo, exactly as is observed 

 With the eggs of the Arachnoidae in general. — Ed. 



