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. 



Ir is this specially noticeable 



part of the larval body thai 



lias been called the velum. 



it can, in later stages, be 



retracted within the shell 



by special muscles (ventral 



and dorsal retractor muscles, 



Fig. 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, 

 stomaeli ; s, shell; sm, anterior adductor 

 muscle; ss, shell-hinge; Vet, velum; vrn, 

 ventral retractor muscle. 



The velum is such a powerful locomotors 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 Lamelli branch larvae seem to leave the egg-envelope at a very early 

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

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

 hfe. Tins latter is the case with the American Oyster and Modiolaria as well 

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



D 



