OF MASSACHUSETTS. 43 



the mantle. There are two kinds: (1) the large, highly extensible tenta- 

 cles, lining the outer edge of the mantle, called by the writer " mantle 

 tentacles " to distinguish them from (2) the inner or " guard tentacles," 

 which lie on the edge of the perpendicular mantle flap. The " mantle 

 tentacles " comprise several rows, apparently without any definite 

 arrangement in the adult. When extended they have the appearance 

 of long, slender white bars covered with minute conical projections, 

 each tipped with a hair. The " guard tentacles " differ from the former 

 in extensibility and function. They extend nearly the entire edge of the 

 mantle flap, except in the region of the two siphonal openings below 

 the " ears," and evidently act as strainers to keep out foreign substances 

 from the mantle chamber. 



The first specialization of the mantle border, the tentacles, appear 

 when the growing condition of the animal demands sensitory functions. 

 They appear soon after the scallop passes the size of .5 of a millimeter, 

 just previous to phase 5, when they can be seen fairly well developed. 

 The first tentacles were noted as conical papillary projections .04 of a 

 millimeter in height, tipped with single cilia (Fig. 44a) on the border 

 of the ciliated mantle. Soon another rises close to the first, or more 

 likely there is a division into two with a granular core between (Fig. 

 44b). The growth continues by repeated subdivisions and the extension 

 of the core part of the mantle until a colony of these projections is 

 formed (Fig. 45), covering a single tube of blood spaces, nerves and 

 tissue, the tentacle proper. The papillary projections radiate from the 

 stalk in such a manner as to give it the appearance of a pineapple 

 (Fig. 46). Such projections are noticeable on the tips of the tentacles 

 during stage 5 (Fig. 29). 



The first tentacles to form are in the ventral region of the mantle 

 (Fig. 27). When the young scallop has nine large tentacles on each 

 mantle lobe, it has seven eyes, which alternate with tentacles. At this 

 stage there are nine slight secondary tentacles which arise between the 

 large ones and in definite relation to the eyes (Fig. 29). As the scallop 

 gTows the tentacles increase rapidly by this method of interpolation, with 

 the result that there finally is apparently no definite arrangement of 

 tentacles and eyes. The first nine tentacles may be styled primary, as 

 they are much larger than the others, which, taken in the order of their 

 occurrence, are called secondary, tertiary, etc. It is interesting to note 

 (Fig. 29) that no primary tentacle is near the central region of the 

 pseudo-siphon, but that there is one on each side. In scallops of 1% 

 millimeters these tentacles when extended measure two-thirds the height 

 of the animal. The " guard tentacles," on account of their function, 

 are quite different in appearance from the "mantle tentacles," being 

 less extensible and heavier. 



There are several uses for the tentacles of the young scallop, especially 

 the primary, which are not functional in the adult. In floating, the 

 small animal opens the shell, extends the tentacles to fuU length, and. 



