<S?. -S, o. 



'H- 



vise. 



Microns 



100 



Figure 341. — Diagram of the nervous S}'stcm and sense 

 organs of fully developed larva of 0. edulh. According 

 to Erdmann, 1935. a.s.o. — apical sense organ; e.g. — 

 cerebral ganglion; ey. — eye; ped.g. — pedal ganglia; 

 pl.g. — pleural ganglion; stc. — statocyst; visc.g. — visceral 

 ganglia. 



342 shows the structure of the larva as it appears 

 in the narcotized live specimen. The drawing is 

 a composite from a number of photographs of 

 live larvae taken with the microscope magnifica- 

 tion of about 100 X, and from examination under 

 higher power of specimens mounted in glycerin 

 jelly. Only the organs visible under these con- 

 ditions are shown in this illustration. The larvae 

 were at the last stage of development, over 300/u 

 in height, with eyes (ey.) and a well-developed 

 foot (f.). The velum was large with long cilia at 

 the top and a row of shorter ones forming an 

 aboral circle (ab.c.) at the base. The apical 

 organ could not be seen in the whole mount prep- 

 arations. The retractors of the velum (r.v.) 

 were well developed. As in 0. edulis they con- 

 sisted of rapidly contracting bands of striated 

 fibers. When the velum is completely withdrawn 

 within the shell cavity, the valves close and the larva 

 drops to the bottom. In a contracted state the 

 different organs become undistinguishable. The 

 well-developed foot (f.) contains a large byssus 

 gland (b.g.). During swimming it protrudes be- 

 tween tlie valves and is kept in the direction of 

 swimming. The tip of the foot frequently turns 



right or left and up and down while the larva is 

 swimming. This behavior suggests that it serves 

 to orient the movements. At the last phase of 

 larval life the foot is used for crawling over the 

 hard surface where the young oyster will finally 

 attach itself. The funnel-shaped mouth (m.) 

 leads to a narrow and long esophagus (e.), which 

 opens into a barrellike stomach (st.) partially 

 surrounded with massive and dark digestive diver- 

 ticula (d.div.). The intestinal loop (int.) and 

 rectum (r.) are similar to those described for 

 0. edulis. Both adductor muscles (ant.ad. and 

 post. ad.) are well developed. The gill rudiment 

 (g.) appears as a strand of cells in the mantle 

 cavity, and the beating of the heart (h.), located 

 between the stomach and the posterior adductor, 

 can be seen in live specimens. At 24° C. the 

 beating of the heart is rapid, varying from 80 to 

 100 pulsations per minute. 



Food apparently is gathered by the ciliary 

 mechanism of the velum, and small food particles 

 can be observed entering the esophagus and 

 moving inside tlie stomach wliere they are rotated 

 by the ciliary epithelium. The ciliated apparatus 

 of the gills has not yet fully developed, and food 

 is gathered only by the aboral circle of the velum 

 (ab.c.) and by the labial palps around the mouth. 

 The statocysts (stc.) and the eye (ey.) are well 

 formed. In a tangential section the eye of C. 

 tirtjimca appears as a transparent lens surrounded 

 by a circle of darkly pigmented cells (fig. 343). 

 The dark liand is a short branch of a nerve leading 

 to the eye. 



The highly developed ciliary mechanism of the 

 velum and the rapidly contracting velar retractors 

 are essential to the life of a free-swimming larva. 

 Their structure appears to be better developed 

 than those of the muscles and ciliary epithelium 

 of adult oysters. Electron microscopy reveals 

 that the ciliated cells of the velum ha\'e a highly 

 complex system of basal bodies and rootlets with 

 distinct periodicity (fig. 344). Intercommunica- 

 tion between adjacent cilia through the basal 

 bodies and their branches pro\-ides a system for 

 tlie coordination of ciliary motion. The com- 

 plexity of the ultrastructure conforms to the com- 

 plexity of the ciliary activity of the velum, making 

 it possible for the larva to swim in any direction, 

 to turn around, or instantaneously to stop ciliary 

 activity. The ciliated cells of the velum are very 

 large; their surface is covered with microvilli, and 



LARVAL DEVELOPMENT AND METAMORPHOSIS 



361 



