$ 200. THE ACEPHALA. 219 
They surround the coils of the intestine, and often ascend along the back, 
covering the liver with their folds, Their excretory ducts are lined with 
ciliated epithelium, and open each side of the bottom of the abdomen 
through a fissure with smaller borders, communicating either with the cav- 
ity of the mantle close by the renal opening, or with the renal sacs.°? With 
those species which have a very small abdomen, these organs are spread out 
by numerous ramifications into the substance of both halves of the mantle.“ 
The two external branchiae serve, for the most part, the function of an 
uterus ; for the eggs, having escaped from the oviduct, are lodged in their 
compartments, and, by the aid of the cilia covering the cavity of the mantle, 
receive the sperm which is introduced in the water for respiration.“ The 
quantity of eggs thus accumulated is so great, that with Anodonta these 
organs are extraordinarily enlarged during the development of the young ;- 
and on this account the shells of the females of this genus are more convex 
than those of the males. In this way the sexes of these Naiades can be 
quickly distinguished from each other by the shell alone. 
§ 200. 
Most of the Acephala undergo during their development, which always 
begins by a complete segmentation of the vitellus, a metamorphosis which 
is quite remarkable in many respects. 
Among the Tunicata, the embryology of the Ascidiae is the best known. 
An oval embryo follows upon the segmented vitellus,® and is quickly 
changed into a Cercaria-like larva.” The tail is not formed froma grad- 
and therefore considered the Naiades as hermaph- 
rodites.* 
12 The genital and urinary openings are cohtigu- 
ous with the Naiades ; see above § 196, note 3, and 
Neumann, De Anodontarum et Unionum oviductu. 
Diss. Regiomont. 1827. This is the same also with 
Tichogonia ; see Van Beneden Ann. d. Sc, Nat. 
VII. 1837, p. 128. With Pinna nobilis, I have 
found the genital orifice close behind those of the 
renal sacs. According to Garner (loc. cit. p. 92), 
a similar arrangement exists with T'ellina, Car- 
dium, Mactra, Pholas, Mya, and Pecten. The 
two genital orifices which Valenciennes (Arch. 
du Mus. &c. I. Pl. I. fig. 5) and Delle Chiaje 
(Descriz. &c. IIT. Tav. XC. 
lower end of the abdomen of Panopaea and Solen, 
belong probably also to the urinary system. 
18 Mytilus (Poli, loc. cit. II. p. 202, Tab. XXXI. 
fig. 8), Anomia, Hiatella, Modiola, and Lithodo- 
mus (Garner, loc. cit. p. 97). With Lithodomus 
dactylus, I have, however, always found the abdo- 
men filled with testicular or ovarian masses. 
14 It is with the Naiades that the branchiae as 
reservoirs of eggs, are best known ; see Poli, loc. 
cit. I. p. 5, Tab. IX. fig. 18; Pfeiffer, loc. cit. 
Abth. If. p. 11, Taf. II. fig. 16-18; Carus, Nov. 
Act. Acad. &c. p. 17, Taf I. fig, 8; and Neu- 
wyler, loc. cit. p. 18, Taf. ILI. fig. 14 (Unio and 
Ae While remaining in the compart- 
ments of the branchiae the eggs are slightly glued 
together. With Unio, they often escape through 
* [ § 199, note 11.] The hermaphrodite character 
of the Naiades has recently been urged, and Kirt- 
dand’s marks of the different sexes by the shape of 
the shell called in question ; see article Zoology in 
the Iconographic Encyclopzdia, edited by Spencer 
F. Baird, p. 70. But see Kirtland’s criticism of 
fig. 2) have seen at the, 
the anal fissures under the form of oval discs 
haped like the branchial compar For along 
time it was inexplicable how the eggs should al- 
ways pass exclusively into the external branchiae, 
when the cavities of the internal ones were so 
much nearer the genital orifices. But Baer 
(Meckel’s Arch. 1830, p. 313) has shown that 
their route is circuitous ; they glide along the base 
of the internal branchia to the cloaca, they then 
ascend by a special canal of the mantle and pass 
into the external branchiae. This course is the more 
easily understood from the ciliated structure of 
these organs. Will (Froriep’s neue Not. No. 620, 
p. 57) affirms that with T'edlina the sperm of the 
males ig evacuated in an analogous manner through 
the anal tube, and being there taken up by the 
females through their respi 'y tube is ducted 
to the external branchiae. 
I have also found embryos within the branchiae 
of Teredo navalis. 
15 At present, this difference in the convexity of 
the valves appears to me to exist only with Ano- 
donta (Wiegmann’s Arch. 1837, I. p. 415); but 
Kirtland (Ibid. 1836, I. p. 236) has succeeded very 
wellin distinguishing, by this character, the males 
and females of Unio, of North America. 
1 The complete segmentation of the vitellus has 
been observed by Milne Edwards (Sur les: Ascid. 
&e. p. 30, Pl. IV. fig. 1-4) with the eggs of Ama- 
Toucium. ~ 
2 These Cercaria-like foetuses had already been 
this article in the Proceed. Amer. Assoc. Advancem. 
of Sc. 5th meeting, Cincinnati, 1851, p. 85. I have 
examined this subject with some care by the micro. 
Scope, and have satisfied myself from an analysis 
of the contents of the organs that the genera in 
question are of separate sexes. — Ep. 
