§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 niantle.'^''^ 

 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 Anodonia 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-Iike larva.'-* The tail is not formed from a grad- 



and therefore considered the Naiades as hermaph- 

 rodites.* 



1- The genital and urinary openings are contigu- 

 ous with the Naiades ; see aljove § 196, note o, and 

 Neumann, De Auodontarum et Unionum oviductu. 

 Diss. Regiomont. 1827. Tliis is the same also with 

 Tichos:onia ; see y'an Beneden Ann. d. Sc. Nat. 

 VII. 1837, p. 1-28. With Pinna nobilix, 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 Tellina, Car- 

 dium, Mactra, Pholas, Mya, and Pecten. The 

 two genital orifices which Valenciennes (Arch, 

 du Mus. &c. I. PI. 11. fig. 5) and Delle Ckiaje 

 (Descriz. &c. III. Tav. XC. fig. 2) have seen at the 

 lower end of the abdomen of Panopaea and Solen, 

 belong probably also to the urinary system. 



V'Mytilus {Poll, loc. cit. II. p. 202, Tab. XX.XI. 

 fig. 3), Anomia, Hiatella, Modiola, and Lithodo- 

 nius (Garner, loc. cit. p. 97). With Lithodomus 

 dactylus, I have, however, always found the abdo- 

 men filled with testicular or ovarian masses. 



'■• 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 ; Pfeijfer, loc. cit. 

 Abth. II. p. 11, Taf. II. fig. 16-18 ; Carus, Nov. 

 Act. Acad. &c. p. 17, Taf I. fig. 8 ; and Neu- 

 viyler, loc. cit. p. 18, Taf. III. fig. 14 {Unio and 

 Anodonia). While remaining in the compart- 

 ments of the branchiae the eggs are slightly glued 

 together. With Unio, they often escape through 



the anal fissures under the form of oval discs 

 shaped like the branchial compartments. For a long 

 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 

 (MeckeCs 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 e.xternal branchiae. This course is the more 

 easily understood from the cihated structure of 

 these organs. Will (Froricp's neue Not. No. 620, 

 p. 57) affirms that with Tellina the sperm of the 

 males is evacuated in an analogous manner through 

 the anal tube, and being there taken up by the 

 females through their respu'atory tube is conducted 

 to the e.xternal 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- 

 donia (Wiegmann''s Arch. 1837, I. p. 415') ; but 

 Kirtland (Ibid. 1836, I. p. 236) has succeeded very 

 well in 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 Edward.i (Sur les Asckl. 

 &c. p. 30, PI. IV. fig. 1-i) with the eggs of Ama- 

 roucium. 



'■i These Cercaria-like foetuses had already been 



* [ § 199, note 11.] The hermaphrodite character 

 of the Naiades has recently been urged, and Kirt- 

 land''s marks of the different sexes by the shape of 

 the shell called in question ; see article Zoology in 

 the Iconographic Encyclojia^dia, edited by Spencer 

 >'. Baird, p. 70. But see Kirtland's criticism of 



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 iii 

 question are of separate sexes. — Ed. 



