THE TRANSFORMATION INTO THE ADULT. 45 



make their appearance on the outer surface of the gill-plate, and a 

 corresponding series also appears on the inner aide of the fold. 



These grooves deepen until individual members of the outer series 

 meet and fuse with the corresponding grooves of the inner series. 

 Perforation takes place along this line of fusion, and in this way 

 gill-slits arise which lie vertically to the longitudinal axis of the 

 branchial lamella. As the grooves start from the extreme ventral 

 edge, the slit is open below, and the branchial plate becomes broken 

 up into filaments (Fig. 21 B). 



We have here described the way in which the gills arise in the 

 two Lamellibranchs whose ontogeny happens to be best understood, 

 but this description does not apply to all Lamellibranchs ; indeed, it 

 is even probable that the condition described above is a specialised 

 one. Thus, in various Lamellibranchs which possess a typical Trocho- 

 I>IK>I-<> larva, e.y., Mytilus, Dreissemia and Ostrea, the gills arise as 

 a row of papillae consecutively arranged, which become subsequently 

 connected together to form the gill-lamellae. The formative pro- 

 cesses which take place in these cases will be detailed below (p. 68). 



[A third method which occurs in Scioberetia (BERNARD, No. I.) 

 and Phola* (SINGERFOOS, No. V.) somewhat resembles that seen in 

 Cyclas and Teredo. As in the latter, a gill-plate first appears, but 

 the gill-slits do not at first extend to the ventral edge of the fold, 

 consequently, perfectly distinct gill-filaments are not formed, but 

 only gill-bars alternating with slits, the gill-plate retaining its 

 original continuity below the slits.] 



In connection with the external form of the Lamellibranchia we 

 have still to mention the labial palps (oral lobes). These, in the 

 adult, are divided into an upper (anterior) and a lower (posterior) 

 pair. In Cyc.las, according to ZIEGLER, they arise in the following 

 manner. The ciliated area surrounding the mouth becomes divided 

 into an upper and a lower portion by a groove which runs out on either 

 side from the angle of the mouth. The first of these must be reck- 

 oned as the upper and the second as the lower lip. These two areas 

 by further growth give rise to the labial palps. At the time when 

 the mantle grows down over the upper lip, a median depression ap- 

 pears in the latter, and a similar depression is to be found in the 

 lower lip, each of the lips being thus divided into two lateral portions. 

 These now begin to grow out as folds, and develop into the labial 

 palps. 



[In most Lamellibranchs the two halves of the upper and lower 

 lips are connected by bridges above and below the mouth, the divi- 



