§200. 



THE ACEPHALA. 



221 



extends, while its opposite end appears attached near the nucleus to the 

 dorsal portion of the walls of the body.'"> 



The solitary individuals, which are produced by the chain-like forms, 

 are also developed near the nucleus, and adhere quite singularly to the 

 dorsal wall of the parent by a peduncle resembling an umbilical cord. 

 These pedunculated embryos are always few in number, and have a proper 

 vitellus. Often, however, there is only one.'^' It may be questioned, 

 moreover, if these eggs are not gradually developed with their peduncle at 

 their place of incubation, or whether they become fixed at this place after 

 having been developed in, and separated from the ovary. At least, one 

 might almost think that, after all, this is only an internal gemmation. 



With the Lamellibranchia, the Naiades particularly, are those whose 

 embryology has been observed. When the vitellus begins to segment, there 

 are two superficial contiguous vitelline cells that do not participate in this 

 process.'-'* These are gradually changed into two three-sided valves, while 

 the remaining portion of the vitellus is transformed into a round embryo 

 covered with cilia, which turns upon itself in the egg — being partly 

 enveloped by the valves.'^"* This rotatory movement, however, soon 

 ceases, and the embryo divides itself into halves, each covered by a valve. <^^' 

 Each of these valves has a ciliated mouth near the hinge, and a proper 

 intestinal canal. <^-' 



In the middle of the angle formed by these halves is raised a short, hol- 

 low cylinder, — the byssus-forming organ, and out of which projects a very 

 long transparent byssus.'^'^' 



(") See the figures of Ckamisso, loc. cit., of Quoy 

 jiiiil Gnhnard (Ann. d. Sc. Nat. X. 1825, p. 226, 

 PI. VIII. fig. 3-6, and A^jyage de I'Astrolabe loc. 

 cit.), of Velle C/iinje (Descriz. &c. III. p. 42, Tav. 

 LXXVI. fig. 1), and especially those of Eschricht 

 (loc. cit. p. 35, Tab. I. II. IV. V.). 



(8) See C/inmixso, loc. cit. flg. 1, D. 1, J. (5a/- 

 pa pinnata^ with -J, very developed foetus), fig. 3, 

 ¥. (Salpa zonnria vrith three button-like foetuses 

 little developed), Quoi/ and Gaimard, Isis, 1836, 

 Taf. VI. fig. 12 {Salpa pinnata, with a very large 

 foetus), and Ami. d. Sc. Nat. loc. cit. PI. VIII. 

 flg. T-9 {Salpa microstoma^ with four button- 

 like foetuses) ; Meyen, lix;. cit. p. 399, Tab. 

 X.WII. fig. 9-10 (Salpa pinnata), Tab. XXVIII. 

 fig. 1, 2 {Salpa mucrnnata). Tab. XXIX. fig. 1. 

 h. (Salpa antarctica), flg. 2-4 (Salpa maxima); 

 Eschricht, loc. cit. p. 65. flg. 27, q. 36 (an individ- 

 ual from the chain of Salpa cordiformis contain- 

 ing flve, isolated, pedunculated foetuses ; perhaps 

 here should be mentioned the five pedunculated 

 bodies which he (p. 39, fig. 18, p. 23) has de- 

 Bcril)ed and figured with Salpa zonaria) ; finally 

 Delle Chiaje, Descriz. &c. loc. cit. Tav. L.YXVIII. 

 fig. 3 (Salpa maxima, with a pedunculated body), 

 and fig. 8, 13 (Salpa scuti^era, with a developed 

 foetus). 



'■> These may be easily distinguished with Unio 

 and Anodonta, a clear nucleus in each segmented 

 division of the vitellus. Cams (Nov. Act. Acad, 

 p. 43, Tab. II. fig. 1, 3, 10, 11) has seen the vitel- 

 line cells nucleated, but he thought that the eggs, 

 which contained the faceted cells out of which are 

 ultimately formed the valves, were diseased and 

 dead. 



'0 This rotation of the embryo of moUusks had 

 already excited the astonishment oi Leeuwenhoek 

 (Continual, arcan. nat. Epist. 95). Its cause was 

 explained in a very unsatisfactory manner by 

 Home (l»hil(js. Trans. 1827, pt. I. i>. 39, or in Hen- 

 singer's Zeitsch. fur organische Pliysik. I. p. 394), 



19* 



and by Cams (loc. cit. p. 27), for they did not know 

 of the existence of cilia. 



n This division of the embryo with valves often 

 lying entirely open, has perhaps led Rathki 

 (Schrivter af Naturhist. Selsk. loc. cit. p. 166, Tab. 

 X. fig. S), and Jacobson (Observ. sur le develop, 

 pretendu des oeufs des Moulettes ou Unio et des 

 ^nodontes dans leurs branchies, An. d. Sc. Nat. 

 XIV. 1828, p. 22, and De BlainvilleKi report upon 

 this work) to regard the young of Naiades for 

 parasites, under the name of Glochidium parasiti- 

 cum. See also the works of Cams (loc. cit.), and 

 of Quatrefages (Sur la vie interbranchiale des 

 petites Anodontes, Ann. d. Sc. Nat. IV. 1835, p. 283, 

 V. 1836, p. 321, PI. \l\.). 



Loven informs us that the young of Modiola, 

 and Kellia are formed upon a wholly different 

 type. Their two valves, which are only slightly 

 separated, are overlapped V>y two lobes (as Man- 

 tle .') which are everted and provided with very 

 active vibratile cilia ; the young swim by means 

 of these lobes (Arch, skandiiiav. Beitr. Kur Natur- 

 gesch. Th. I. p. 155, Taf. I. fig. 9-11). I also have 

 oljserved with Teredo navalis, the embryos swim- 

 ming freely about by means of a foot-like organ 

 which protrudes betweeu the valves and presents 

 an active ciliary movement. 



12 Quatrefasres, loc. cit. PI. XII. fig. 20. 



13 ^uatrefages (loc. cit.) has figured with 

 each embryo two byssus-organs out of which pro- 

 jects a double byssus. I have been unable to ob- 

 serve this, and, like Cams, have always found a 

 single byssus-organ with a single byssus. 



It is quite remarkable that not only the Naiades, 

 but other Lamellibranchia also, have this byssus- 

 organ when quite young. Thus in the young of 

 Cyclas cornea, I have distinctly seen a hollow 

 pyriform glandular organ in the foot, and from 

 which projected a long simple byssus. 



The embryos of Kellia have also a byssus ac- 

 cording to Loven (loc. cit.). 



