42 NILS HJ. 0DHNER, STUDIES ON RECENT CHAMIDAE. 



rectly opposite to each other throughout their entire extent, though they are se- 

 parated by the space named. On their outer and back sides they are covered by 

 the exteriör sacs. These are minutely and densely lobed, though not so strongly 

 as in the full-grown animal. The nephroprocts are situated, as usual, out of and 

 beneath the nephrostomes and separated from the genital pores. At this stage weak 

 rudiments of posterior retractors exist. 



In the full-grown animal the conditions prevailing in the young have altered 

 in the following manner (figs. 44, 45). The nephridia become closely compressed 

 against each other, so that the pericardial tubes lie side by side and the outer sacs 

 in front of and behind them are adjacent. Here, too, as a consequence, they still 

 retain the primitive character of free pericardial tubes, not surrounded by the outer 

 sacs. Shortly behind the point where the pericardial tubes diverge, some small lobes 

 of the outer sacs protrude between them and extend behind the portions of them 

 that run on the hinder side of the outer sacs (fig. 44). In the young specimen no 

 similar lobes exist, and the pericardial tubes run superficially on their inner sides 

 throughout. Further, in the full-grown animal there is an ampulla-shaped dilatation 

 of the pericardial tube developed behind the funnels and the entire pericardial tube 

 is relatively somewhat wider than in the young one. The outer sac is strikingly 

 lobed by means of high folds. The position is also changed inasmuch as the peri- 

 card has been greatly compressed in an antero-posterior direction, probably as a 

 consequence of the augmentation in space of the genital glands; thus, in its lower 

 end, only the lateral corners are retained, and these are pressed farther laterally 

 than in the young animal. The nephroprocts are situated beneath the genital pores, 

 separated from and opposite to them. Every trace of the foot retractors which 

 appear distinctly in the earlier stage is now absent; there is only a strong muscular 

 layer covering the isthmus in front of the nephridial openings. 



Genital organs. In the young specimen the liver occupies the anterior, the 

 genital glands the posterior part of the visceral hump, though in its upper part 

 these organs extend their branches as far as the mouth. The extensive development 

 of the genital organs shown in the adult stage seems to be the cause of the sub- 

 sequent compression of the pericard, a process that is observable in many genera 

 of Lamellibranchia, as I have pointed out in my publication on the nephridia and 

 their phylogeny (in 1912). 



Pseudochama retroversa Lischke. 



Figs. 46—49. 



The two specimens examined, one of them in sections, were collected in Labuan, 

 Borneo, by the Vega Expedition in 1879. 



