PIIYLOGENY OF THE PELECYPODA. 341 



scallop, the valves are less widely separable, when open, and at time of danger stay shut, 

 instead of opening- almost immediately after closure, as with scallop shells; therefore, the 

 oyster is less liable to attack, at least from certain classes of enemies, than is the scallop. 

 . A surprising want of sensibility is noticeable in adult Pectens. It is often possible 

 to insert a quite large body into the shell before it closes; but the young are more sen- 

 sitive. Frequently pricking the mantle border with a needle produces little effect in 

 adults. Both adults and young are sensitive to light, so that passing the hand or even 

 a pencil in front of the valves they will quickly close, as described in Pecten jacobmus 

 by Patten. They open almost immediately, however, and frequently repeated stimuli soon 

 lose their etfect. 



Pecten irradlans is hermaphroditic, the reproductive organ is of large size, and the 

 two sexual functions are distributed regionally. The gland as a whole is roundly tri- 

 angular, extending between the gills on the ventral border of the adductor muscle, while 

 anteriorly it passes into and is lost in the liver mass. The gland is composed of two dis- 

 tinctly marked portions or ai-eas. The ventral area is pinkish white and consists whol- 

 ly of the male or sperm-bearing portion; the dorsal area is brownish-yellow in color and 

 consists of the female or egg-bearing portion of the gland. The divisions of the organ 

 lie parallel, but anterioi-ly the'whole mass is male, while the female portion occupies the 

 larger part of the posterior portion of the organ. The whole gland is more or less pig- 

 mented with black, especially on its ventral keel. Abundant active spei-matozoa were 

 found in the pinkish or male portion, with well developed heads and vibratile tails; but no 

 eggs, it being solely spermatic. In the yellowish-brown dorsal portion were found de- 

 veloping and ripe eggs. The e^g,^ are rounded or pear-shaped, granular, show nuclei and 

 resemble the egg of an oyster, in size and appearance. A few spermatozoa were found 

 in the female portion, but their presence was probably due to accidental introduction 

 while examining the contents of the gland, as it is almost impossible to avoid introduc- 

 ing a few luider such circumstances, and we may fairly suppose that the female portion 

 also produces but one element. The sexual gland of Pecten irradimis, it will be seen, is 

 very similar to that of P. glaher, as described by Lacaze-Duthiers (40). 



The gills of adult Pecten irradians are equal in length and ditfer much from the gills 

 of the oyster. The filaments of the gills are reflected on themselves to such a length 

 that the reflected portion equals about two-thirds the length of the direct portion as shown 

 diagramniatically in PI. xxiv, fig. 12. The reflected borders are free, not joined by con- 

 crescence with the reflected borders of opposing gill filaments, or with the mantle lobes, 

 as in Ostrea. The filaments are joined to one another by the interlocking of tuft-like, 

 ciliated nodes, situated at intervals. About 25-30 nodes are borne on each filament, 

 and the nodes of contiguous filaments are in apposition. A similar structure exists in 

 Mytilus, and is described by Peck. This connection does not exert a very strong bind- 

 ing influence, so that when alive the filaments of Pecten are often separated, and in alco- 

 holic specimens are notably disconnected. The ciliated nodes serve to keep the filaments 

 together, but quite as much to kee]) them sepai-ated, by an interspace to allow of the free 

 passage of water. In the adult Pecten, at intervals of every 11-15 filaments, a delicate 

 eonnccling septum or wall is developed between the direct and reflected border of the 

 filaments, PI. xxviii, fig. 3. These walls strengthen the reflected borders and also 



MCSIOIIiS BOSTON SOC. XAT. HIST., VOL. IV. IG 



