THE FORMATION OF THE ORGANS THE GENITAL ORGANS. 217 



shifts quite to the front, the gill will be found in from of the 

 heart and the auricle in front of the ventricle (Prosobranchia). 



Other descriptions of the rise of the pericardium, the kidney and the 

 heart." In the formation of the pericardium as described above, this organ 

 wa- treated as if it corresponded to the whole of thecoelom, but v. Erlanger's 

 rvations on Paludina and Bythinia may also be interpreted as showing 

 only a pan of the original coelom persists as the pericardium while the 

 rest disintegrates, as we saw to be the east- in the formation of the definitive 

 body-cavity in the Arthropoda. Salenskv also, at a somewhat later stage of 

 the embryos of Vermetus, speaks of a somatic and a splanchnic layer which 

 are apposed to the ectoderm and the entoderm respectively and which enclose 

 a large space as a (temporary) secondary body-cavity. The two layers of tin 

 mesoderm are, however, so indistinct in the Mollusca that we are unable to 

 -peak of them with any certainty and, until more detailed statements are 

 made, must regard them as only definitely differentiated in the pericardium. 

 Sajjsssky, who regarded this large space as the coelom, considers that the 

 heart arose from it in a way similar to that above described. With this may 

 he reconciled the earlier accounts of Ganin {No. 35), Butschli (No. 18) and 

 especially of P. Sarasin (No. 101) and Schalfeew (No. 106) which refer 

 partly to the Prosobranchia and partly to the Pulmonata. 



It is easier to reconcile the older and more recent researches with regard 

 to the rise of the heart than with respect to the origin of the kidney. This 

 organ was indeed early derived from the mesoderm by constriction from the 

 :ardium (Schalfeew) or at least in the neighbourhood of the latter 

 (Salbnsky), the efferent ducts being derived from an (ectodermal) invagina- 

 tion of the mantle-cavity, but the majority of authors trace back the whole 

 kidney to an ectodermal invagination. After what has been said above 

 (p. 74) as to the formation of the nephridia in the Lamellibranchia and the 

 Auuelida, it cannot he doubted that the first method is the more probable. t 



H. The Genital Organs. 



The development of the genital organs has been best observed in 

 Palvdina, a form belonging to the Prosobranchia in which the sexes 

 are distinct (v. Erlangek). In these animals, the condition of the 



*The literature connected with the formation of the mesodermal organs is, 

 like that connected with the ontogeny of the Gastropoda in general, rich in 

 contradictory statements. Where recent researches may be considered to 

 have disproved older statements, we have ignored the latter. Lack of space 

 Da- prevented us from taking into consideration all the published data of a 

 confirmatory nature. A summary of these is to be found in v. Erlanger's 

 works (Nos. ->1 and 28). 



+ [The recent investigations made by Meissenheimeb (No. XVII.) on the 

 development of these organs in Limax do not at present help to clear up the 

 confusion relating to their origin, as they are too startling to be accepted 

 unsupported. Meissenheimer maintains that the heart and kidney arise from 

 a common ectodermal rudiment, a condition which, so far a- we are aware. 

 appears to lie quite unique. — Ed.] 



