$§ 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 Chamisso, loc. cit., of Quoy 
and Gaimard (Ann. d. 8c. Nat. X. 1825, p. 226, 
Pl. VIII. fig. 3-6, and Voyage de l’Astrolabe loc, 
cit.), of Delle Chiaje (Descriz. &c. III. p. 42, Tav. 
LXXVI. fig. 1), and especially those of Eschricht 
(loc. cit. p. 35, Tab. I. IL, IV. V.). 
(8) See Chamisso, loc. cit. fig. 1, D. 1, J. (Sal 
pa pinnata, with a very developed foetus), fig. 3, 
. (Salpa zonaria with three button-like foetuses 
little developed), Quoy and Gaimard, Isis, 1836, 
Taf. VI. fig. 12 (Salpa pinnata, with a very large 
foetus), and Ann. d. Sc. Nat. loc, cit. Pl. VIII. 
fig. 7-9 (Salpa microstoma, with four button- 
like foetuses); Meyen, loc. cit. p. 399, Tab. 
XXVII. fig. 9-16 (Salpa pinnata), Tab. XXVIII. 
fig. 1, 2 (Salpa mucronata), Tab. XXIX." fig. 1. 
h. (Salpa antarctica), fig. 2-4 (Salpa maxima) ; 
Eschricht, loc. cit. p. 65. fig. 27, q. 36 (an individ- 
ual from the chain of Salpa cordiformis contain- 
ing five, isolated, pedunculated foetuses ; perhaps 
here should be mentioned the five pedunculated 
bodies which he (p. 39, fig. 18, p. 23) has de- 
scribed and figured with Salpa zonaria) ; finally 
Delle Chiaje, Descriz. &c. loc. cit. Tav. LX XVIII. 
fig. 3 (Salpa maxima, with a pedunculated body), 
and fig. 8, 18 (Salpa scutigera, with a developed 
foetus). 
9 These may be easily distinguished with Unio 
and Anodonta, a clear in each seg d 
division of the vitellus. Carus (Nov. Act. Acad. 
p. 43, Tab. IL. 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. 
10 This rotation of the embryo of mollusks had 
already excited the astonishment of Leeuwenhoek 
(Continuat. arcan. nat. Epist. 95). Its cause was 
explained in a very unsatisfactory manner by 
Home (Philos. Trans. 1827, pt. I. p. 39, or in Heu- 
singer’s Zeitsch. fiir organische Physik. I. p. 394), 
19%* 
and by Carus (loc. cit. p. 27), for they did not know 
of the existence of cilia. 
11 This division of the embryo with valves often 
lying entirely open, has perhaps led Rathké 
(8chrivter af Naturhist. Selsk. loc. cit. p. 166, Tab. 
X. fig. 3), and Jacobson (Observ. sur le develop. 
prétendu des oeufs des Moulettes ou Unio et des 
Anodontes dans leurs branchies, An. d. Sc. Nat. 
XIV. 1828, p. 22, and De Blainville’s report upon 
this work) to regard the young of Naiades for 
parasites, under the name of Glochidium parasiti- 
cum. See also the works of Carus (loc. cit.), and 
of Quatrefages (Sur la vie interbranchiale des 
petites Anodontes, Ann. d. Sc. Nat. IV. 1835, p. 283, 
V. 1886, p. 321, Pl. XII.). 
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 by two lobes (as Man- 
tle 2) which are everted and provided with very 
active vibratile cilia; the young swim by means 
of these lobes (Arch. skandinav. Beitr. zur Natur- 
gesch. Th. I. p. 155, Taf. I. fig. 9-11). I also have 
observed with T'eredo navalis, the embryos swim- 
ming freely about by means of a foot-like organ 
which protrudes between the valves and presents 
an active ciliary movement. 
12 Quatrefages, loc. cit. Pl. XII. fig. 20. 
18 Quatrefages (loc. cit.) has figured with 
each embryo two byssus-organs out of which pro- 
jects a double byssus. Ihave been unable to ob- 
serve this, and, like Carus, 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 inthe 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.). 
