82 LAMELLIBKANCHIA. 



bution of these in the pseudocoele, and the further growth of the 

 isolated cells into contractile fibres. When considering the larval 

 forms, we pointed out that these fibres become applied to one another 

 to form larger complexes which are the muscles of the larva and the 

 adult (Fig. 15, p. 31 ; Fig. 18, p. 36). The musculature of the foot 

 arises from the great increase in number of the cells detached from 

 the mesodermal mass, and the massive connective tissue both of the 

 foot and of the rest of the body has the same origin. 



H. The Genital Organs. 



The ontogeny of the genital organs has not as yet been sufficiently 

 studied. In Cydas, the genital glands originate from the two meso- 

 derm-bands and lie, as a rather large mass of cells, beneath the peri- 

 cardial vesicle and close under its wall (ZIEGLEB). A somewhat later 

 stage in the development of these glands is depicted in Fig. 21 A, g, 

 p. 44. At a still later stage, they form two club-shaped masses, 

 the broad surfaces of which meet in the middle plane, lying above the 

 oerebro-visceral commissure (Fig. 31, f/). 



From what is as yet known of these glands in the Lamellibranchs, they do 

 not bear any direct relation to the pericardial sacs, i.e., to the epithelium of 

 the secondary body-cavity, as was found to be the case in the Annelids and as 

 we shall presently see that they do in other Mollusca. Our knowledge of the 

 subject is far too slight to justify further conclusions, but we may suggest that 

 the close relation of the coelom to the kidney has led to an alteration in the 

 conditions and thus to a gradual shifting of the genital rudiment out of the 

 coelom. The efferent ducts must at the same time have undergone alteration, 

 but with respect to these points, ontogeny fails us and we can only draw our 

 deductions from the anatomical conditions. 



The relation of the efferent genital ducts varies in the Lamellibranchia. 

 Most usually, they open on the surface of the body independently of the 

 nephridia. In by far the greater number of the Eulamellibranchia they open 

 into the supra-branchial cavity near the external aperture of the kidneys. 

 In other Lamellibranchs, they and the efferent ducts of the kidneys open into 

 a common cloaca (Area, Pinna, Ostrea, Cyclas) ; in others again, they emerge 

 further back in the organ of Bojanus (Anomia, Spondylus, Pecten, Lima), and 

 only in a few primitive forms (Nucula, Solenomya) do the genital products 

 pass into the kidney, not far from the reno-pericardial aperture (PELSENEER, 

 No. 41). 



Distinct efferent ducts for the kidneys and the genital organs are found in 

 such forms as, from their structure, may be considered as phylogenetically 

 younger than the others, while the two organs are connected in those 

 Lamellibranchs which, by their organisation and their early geological 

 occurrence, are proved to be of greater age (v. JHERING). These facts indicate 



