THE TROCHOPHORE STAGE AS A FREE-SWIMMING LARVA. 



33 



the body. This becomes very pronounced at a stage when a slight 



constriction of the body behind the pre-oral ring is found, as in the 



Oyster depicted in Fig. 16. 



It is this specially noticeable il 1 111 



part of the larval body that iJJAU ;,, 



,,f 1 



has been called the velum. 

 It can, in later stages, be 

 retracted within the shell 

 by special muscles (ventral 

 and dorsal retractor muscles, 

 Fir. 16, vm and dm, and 

 Fig. 18), so that the larva 

 appears highly contractile. 

 In the anterior (pre-oral) 

 part, -i.e., in the region of 

 the velum, the larva is 

 often more or less strongly 

 pigmented (Dreissensia), 

 and has thus a peculiar and 

 striking appearance (Fig. 

 17 A). 



.. 



ma. 



FIG. 16. Larva of Ostrea edulis (from RYDER, 

 No. 46, after HUXLEY) ; a, anus ; dm, dorsal 

 retractor muscle ; I, liver ; m, mouth ; ma, 

 stomach ; s, shell ; sm, anterior adductor 

 muscle ; ss, shell-hinge ; \ r el, velum ; vm, 

 ventral retractor muscle. 



The velum is such a powerful locomotory organ that the larva is able to 

 swim with great rapidity in definite directions, and thus does not merely float 

 about in the water like many ciliated larvae. Such a swimming larva, in the 

 position in which it is usually seen at the surface of the water, i.e., with the 

 velum directed upward, presents a very characteristic appearance (Fig. 17 A). 

 The strong covering of cilia carries on an almost continual rowing motion. 

 When the larva is in this position, the whole of the upper part of the body is 

 covered by the velum. The large size of this organ in comparison with the 

 rest of the body can be distinctly seen in older stages of, for example, the 

 Dreissensia larva (Fig. 17 C) in which the massive velum is extended far be- 

 yond the valves of the shell. In this form, the retraction of the velum is 

 assisted by the development of a median groove which divides the velum 

 into two and enables these two cushion-like halves to be folded together. 

 The velum in this way has a peculiar double appearance which is most 

 marked when it is being extended, but is also evident even when it is fully 

 expanded (Fig. 17 B, and Fig. 20). The double velum of the Gastropoda is 

 thus recalled, and the resemblance is much more striking here than in the 

 reduced velum of Cyclas, in which ZIEGLER pointed it out (p. 45). 



Most Lamellibranch larvae seem to leave the egg-envelope at a very early 

 stage, and either remain sheltered within the body of the mother for a long 

 period, like Teredo and the European Oyster, or else at once enter on free 

 life. This latter is the case with the American Oyster and Modiolaria as well 

 as with Mytilus and Dreissensia. The minute and somewhat pear-shaped 



D 



