THE DEVELOPMENT OF THE EMBRYO INTO THE PARASITIC LARVA. 55 



a byssal gland corresponding to the homonymous organ of other 

 Lamellibranchs, but this view, in spite of the similar function of the 

 two organs, is not justified, since the two organs do not agree in 

 position, and since two ectodermal invaginations appear later on 

 the foot of the larva which must be considered as the homologues 

 of the byssal gland (CARRIERE, F. SCHMIDT, SCHIERHOLZ). The 

 glutinous filament must therefore be regarded as a distinct larval 

 organ. 



The position of this filament is very remarkable in so far as it is 

 said to be pre-oral (Fig. 26 A). The mouth has been pressed un- 

 usually far back and, like the intestinal canal (d), now belongs to the 

 small posterior part of the larva. This displacement has been traced 

 to the great development of the adductor muscle (sin), but the 

 morphological conditions of this larval stage as compared with 

 the former Trochophore-like stage seem to us to require further 

 elucidation. 



[LiLLiE (No. III.) describes the thread-gland as arising from one of 

 the cells of the head-vesicle ; this cell elongates and grows backward 

 beneath the hinge-line until it reaches the posterior end of the body. 

 The cell now becomes tubular, the thread occupying the lumen of the 

 gland. LILLIE believes that the thread is formed as an actual meta- 

 morphosis of the substance of the cell ; he regards this gland as 

 primarily excretory, and thinks that the utilisation of its secretion 

 as an attaching filament was secondarily acquired. During the 

 transformation of the larva into the Glochidium, the aperture of the 

 thread-gland undergoes a remarkable change in position, shifting 

 from its former antero-dorsal situation to the middle of the ventral 

 sill-face.] 



Between the brush-like sensory organs and the ciliated area and near the 

 posterior angle of the valves, two ectodermal depressions are to be seen on 

 the embryo; these are the so-called lateral pits, as to the significance of 

 which authors are not very clear. If we rightly understand the somewhat 

 obscure description given by SCHIERHOLZ, he implies that large cells at the 

 base of these pits (no doubt corresponding to the lateral cells of the young 

 embryos) give rise to the pedal ganglia. But since the pedal ganglia, as in 

 Cyclas, lie below the depressions which yield the byssal gland, these early- 

 formed pits may be related to the latter (?). In Cyclas also the paired rudi- 

 ment of the byssal gland appears very early (Figs. 19 and 21). Taking these 

 facts into consideration, together with the position of the pits with relation 

 to the foot, it is possible that this interpretation of the lateral pits as the first 

 rudiments of the byssal glands is correct. The formation of the actual byssal 

 gland in Anodonta, however, seems to take place at a later stage. 



