THE TRANSFORMATION INTO THE ADULT. 43 



These two depressions together with the whole of the ectoderm lying be- 

 tween them soon sink in deeper, having then only one common aperture; 

 their epithelium becomes modified into glandular cells which secrete the well- 

 known byssal threads that serve for the attachment of the embryo or yffung 

 Lamellibtanch (Fig. 21, by). As the body grows further, the paired rudiment 

 of the byssal gland is drawn further and further inward, and finally opens out 

 through a long duct with a narrow aperture. This gland degenerates later 

 (in Cyclas), and in the adult is a mere sac-like vestige. In other Lamelli- 

 branchs, on the contrary, as is well known, it functions throughout life and 

 is very highly developed (cf. p. 59). 



In Entovalva, discovered by Voeltzkow (No. 57) living in the intestinal 

 canal of Synapta, at the posterior margin of the keel-shaped foot, a similar 

 structure was found, which is brought into use as the Lamellibranch moves 

 forward and attaches itself. From its position, it might well correspond to the 

 modified byssal gland but this requires further investigation. In Gastrochaena, 

 a Lamellibranch that inhabits a calcareous tube, there \s an attaching 

 apparatus in the foot consisting of ectodermal depressions surrounded by 

 glandular cells ; the secretion of the latter serves for attaching the broad sole 

 of the foot to the inner surface of the tube, but is said not to correspond to 

 the byssus which, according to Sluiter (No. 53), completely degenerates in 

 this animal. In Entovalva, the foot attains a very high degree of develop- 

 ment and the mantle grows over the shell (as also in Gastrochaena, cf. p. 62) : 

 in other ways the development of this parasitic Lamellibranch is not peculiar. 

 It shows typical Trochophore larvae, which develop in a brood-cavity formed 

 by the mantle ; they pass thence into the intestine of the Holothurian host, 

 reach the exterior with its excrement, and there develop further (no doubt 

 in the usual way): Not until fairly well developed do these young Lamelli- 

 branchs enter the mouth of a Holothurian and pass into the oesophagus. 



At the time when the external form of the foot is already well 

 <leveloped, the ad-oral ciliated area, which has been referred to the 

 ciliation of the Trochophore, is still retained (Fig. 21 A and B) ; the 

 .shell, mi the other hand, undergoes a decided transformation which 

 brings it nearer its adult form. According to ZlEGLER, wdiose ac- 

 count we here follow so far as concerns Cyclas, an accumulation of 

 small calcareous concretions can be seen on each side of the dorsal 

 middle line beneath the unpaired cuticular integument (Fig. 21 .4, s). 

 These masses increase and soon take the form of the definitive shell 

 (Fig. 21 B, s). There is now a shell-plate on each side; the two, 

 however, are not in contact, but are somewhat far removed from 

 the middle line. A band-like thickening of the cuticular shell- 

 integument then develops in this interspace, connecting the dorsal 

 margins of the two shell-valves just below the umbones ; this 

 represents the ligament. 



Before the formation of the shell has advanced thus far. the mantle 

 lias already begun to form. At the two sides of and above the anus 



