OF MASSACHUSETTS. 33 



shown by effervescence when tested with acid. The scallop differs from 

 Anomia glabra at this stage by having no byssal notch in the shell. 



The Velum. — The veliger derives its name from the larval swimming 

 organ or velum peculiar to this period of its life. This organ, situated 

 in the anterior part of the shell, consists of an elliptical pad, .046 milli- 

 meter in length, with a border of short vibrating cilia, and supporting 

 in its center a long flagellum. It is capable of extension and contraction, 

 whereby it can be thrust out of the shell or drawn in quickly by means 

 of retractor fibers, which are fastened to the shell near the posterior 

 part of the hinge, so as to give a direct backward pull. Two fibers 

 go to the ends of the velum, the third to the center. When contracted, 

 the velum folds in to the form of a bell, the round ciUated edges curving 

 toward the central part, which bears the flagellum. When expanded, 

 the velum opens like the unfolding of a flower, the ciliated edges curling 

 outward. When the velum is extended outside the shell, as the animal 

 swims, the whole mass shifts ventrally, leaving a clear space between 

 the hinge and the body. The flagellum serves during this period as a 

 sensitory organ or feeler. The velum is of great use in swimming, 

 and can rapidly propel the young scallop through the water by the 

 lashing of its cilia in a manner similar to the action of oars in a 

 boat. 



The development of the velum can be traced from the ciliated region 

 of the early .gastrula, and the organ is a direct modification of the 

 anterior ciliated area of the trochosphere larva. The frontal cilia, 

 with the long central flagellum, have become more centralized and 

 stronger, while the ciliated arc has formed a muscular pad capable of 

 extension and contraction. The flagellum and cilia of the veliger stage 

 are identical with those of the trochosphere, the only change being a 

 modification of the supporting area. 



While the transition from the veliger to the footed larva has never 

 been completely observed in the scallop, it is doubtless identical with 

 that of the clam, which is here described. This change takes place by 

 the atrophy or degeneration of the velum and the simultaneous develop- 

 ment of the foot. Several stages can be observed during this transition 

 period: (1) the primitive veliger, with no foot or at best a rudimentary 

 projection posterior to the mouth; (2) a reduced velum and a half- 

 formed foot; (3) a small velum and a nearly complete foot; (4) no 

 velum and a perfectly developed foot. During this period the mouth 

 has advanced anteriorly and dorsally, following the disappearing velum, 

 which vanishes in the region of the palps. 



Haiits. — Swimming in the earlier veliger stage is wholly by the 

 velum, whUe later this organ is assisted by the foot. The very young 

 veliger is less active than the older larva, and is usually found at 

 the bottom of the dish with valves widely open and velum partly pro- 

 truded. In this case, the movement merely consisted of turning in a 

 circle, as the velum was not thrust out far enough to enable the animal 



