



30 



Microns 



Figure 336. — Trochopliore of O. edulis according to Horst, 

 1S83. Reproduced from Pelseneer, 1906. a. — anus; e. — 

 esophagus; int. — intestine; m. — mouth; st. — stomach; 

 sh. — shell; pr. — prototroch. 



the veliger stage into a powerful organ for swim- 

 ming. During swimmmg tlie velum projects be- 

 tween tlie valves of the shell. It is highly con- 

 tractile and at the slightest disturbance is with- 

 drawn between the valves by several velar 

 retractor muscles (r.v.), which are attached to 

 the velum and are anchored at the opposite end 

 to the sheU. 



For examination of the velum, the larvae should 

 be narcotized with menthol, chloral hydrate, or 

 other narcotics, and made transparent with 

 glycerol. The larvae can be satisfactorily narco- 

 tized in a small dish l)y placing tiny crystals of 

 menthol on the sm'face of the water and allowing 

 them to relax before giving additional crystals. 

 When narcosis appears to be complete, glycerol 

 should be added slowly, drop by ch'op, to avoid 

 disturbing the larvae and causing th^m to con- 

 tract. The method is tedious, time-consuming, 

 and requires a great deal of patience. 



liarge cilia around the margin of tlie velum are 

 for swuiiming; small cilia (not shown in fig. 337), 

 covering tlie base of the velum carry food particles 

 toward the mouth (m.). A relatively long esopha- 

 gus (e.) leads to a barreUike .stomach (st.), which 

 is in close contact with the glandular structure of 

 digestive diverticula (dig. d.). The crystalline 

 style sac (cr. s.) is at the lower part of the stomacli. 

 The intestine (int.) emerging from the stomach 



makes a single loop and continues into the rectum 

 (r.); the anus (a.) opens into the mantle cavity 

 fm.c). The foot rudiment (f.) appears as a cili- 

 ated outgrowth of the body under the mouth and 

 reaches its full development toward the end of 

 larval life. The anterior adductor muscle (ant. 

 ad.), destined to disappear in older larvae, is con- 

 spicuous; the posterior adductor has not yet 

 developed. 



The early larvae of C. virginica found in plank- 

 ton samples or developed in the laboratory are 

 oval-shaped and shghtly as3Tiimetrical. Because 

 the hinge side of their shells is straight, they are 

 called straight-hinge larvae or D-shaped larvae. 

 Rees (1950) refers to this stage as Prodissoconch I. 

 Dimensions of the larvae vary from 70^ to 75^ in 

 length, i.e., parallel to the hinge side, and from 

 60m to 68m in height, with the greatest distance 

 at a right angle to the hinge side. The prodis- 

 soconchs of C. virginica are shown in the photo- 

 micrographs in fig. 338. 



Major changes take place in the appearance and 

 structure of the larva as it grows, reaches its full 

 development, and becomes ready to set. The ad- 

 vanced stages of larval development are called by 

 various descriptive names referring to the most 

 conspicuous morphological change of each stage: 

 umboned larva, eyed larva, adult, and mature 

 larva. The latter expression is frequently used 



dig. d 



ant. ad 



m.c. 



Microns 



Figure 337. — Early free-swimming veliger of 0. edulis. 

 From Yonge, 1926. a. — anus; ant. ad. — anterior ad- 

 ductor muscle; cr.s. — crystalline style sac; dig.d. — 

 digestive diverticula; e. — esophagus; f. — rudiment of 

 foot; int. — intestine; m. — mouth; m.c. — mantle cavity; 

 r. — rectum; r.v. — velar retractor muscles; sh. — shell; 

 St. — stomach; v. — velum. 



LAR\^AL DE\'ELOPMENT AND MET.-UVIORPHOSIS 



357 



