VII 



MOLL USC A ONTOGENY 



263 



part of each mesoderni streak a group of cells surrounds a cavity, which at first is 

 very small, but becomes continually larger. The two vesicles thus formed, the 

 cavities of which represent the secondary body cavity, form the pericardium. 

 Behind these the mesoderni cells collect in such a way as to form on each side a 

 strand, which becomes hollow ; this is the rudiment of the nephridium, which at 

 once becomes connected with the pericardial vesicle, and, growing further towards 

 the ectoderm, soon opens outward. The two pericardial vesicles lengthen posteriorly 

 and upward, each becoming divided into two parts, one lying behind the other, by a 

 constriction, the parts still communicating dorsally with one another (Fig. 219 A). 

 The two double vesicles grow towards one another above the rectum, and finally 

 fuse in the dorsal middle line (B). In a similar manner they fuse below the 

 rectum. The inner wall of the pericardial vesicle becomes the wall of the ventricle 

 (C), and its lateral wall becomes 

 that of the auricle. At the points 

 where the lateral vesicles were con- 

 stricted lie the slits through which 

 the auricles communicate with the 

 ventricle, and the atrioventricular 

 valves. 



The visceral ganglion arises at 

 the posterior end of the mantle 

 furrow from an ectodermal thicken- 

 ing. The pleurovisceral connectives 

 form, in all probability, throughout 

 their whole length, through con- 

 striction from the ectoderm. The 

 gill arises on each side as a fold on 

 the dorsal edge of the inner surface 

 of the mantle. It develops from 

 behind forward. In the contrary 

 direction furrows form on the 

 branchial fold, commencing from below upwards ; these are found on the inner as 

 well as the outer surface, and exactly correspond. The inner furrows join the outer 

 right through the gill, and thus give rise to the branchial slits. 



3. The development of the Unionidse (Aiwdonta, Unio] is much influenced by 

 the parasitic manner of life of the larva. The fertilised eggs reach the outer leaf 

 of the gill of the female, and there run through the first stages of their develop- 

 ment. Segmentation leads to the formation of a coeloblastula, in which the rudi- 

 ment of the shell gland appears very early as an incurved plate of large and 

 high cells of the blastula wall. The archenteron forms by invagination at a very 

 late stage ; this is no doubt connected with the later parasitism of the larva. 

 Before this invagination occurs the mesoderm has begun to form ; its two primi- 

 tive cells lie in the blastocoel at the part where, later, the enteric invagination 

 appears. 



The embryo known as Glochidium parasiticum has, in the last stage of its 

 development, which is passed through in the gills of the mother animal but within 

 the egg-shell, the following structure (Fig. 220). It is bilaterally symmetrical, and 

 has a bivalve shell. Each valve has, at its ventral edge, a triangular process, 

 the exterior of which is beset with short spines and thorns. Between the two valves, 

 which are markedly concave, lies the soft body, which lines the shell internally in 

 such a way that its ventral epithelial layer might, incorrectly, be called a mantle. 

 It may be called the false mantle. If this false mantle is examined from below, 

 when the shell is open, it is seen to have on each side four sensory cells furnished 



FIG. 219. A-C, Diagrams illustrating the develop- 

 ment of the pericardium and heart of Cyclas cornea 

 (after Ziegler). 1 and 2, The two lateral pericardial 

 vesicles ; 3, rectum ; 4, pericardial cavity ; 5 and 6, 

 invaginations of the lateral Avails of the pericardium = 

 rudiments of the two lateral auricles ; 7 and 8, median 

 walls of the two lateral pericardial vesicles, in B partly 

 fused to form a median septum (above and below the 

 intestine), which in C has disappeared ; 9, rudiment of 

 the ventricle. 



