39 



the lecithotrophic phase extends beyond the Umits of the trochophore stage 

 (Blacknell and Ansell, 1974). The development of lecithotrophic larvae and of 

 brood care are common for small (usually less than 1 cm long) bivalves. If the 

 lecithotrophic larva does not leave the egg capsule before metamorphosis, it 

 loses the adaptation for swimming. Thus, in the larvae of Lasaea rubra (Oldfield, 

 1964), Thyasira gouldi (Blacknell and Ansell, 1974), and Cardiamya pectinata 

 (Gustafson et al., 1986) developing in the egg capsule, the velum loses cilia 

 and becomes a vitalline receptacle. 



Some mention should be made of the unique larvae of the primitive 

 bivalves of superorder Protobranchia (Drew, 1897, 1899a, b; 1901). From the 

 yolk-rich eggs of Nucula delphinodonta and Yoldia limatula, barrel-shaped 

 lecithotrophic larvae develop, swim briefly in plankton. In contrast to most 

 Bivalvia, the prototroch and pretrochal ectoderm are not modified into the 

 velum in these larvae. Growing in size, they cover the entire larval body, 

 forming a temporary larval mantle which consists of five transverse rings of 

 large vacuolated cells. The long cilia of the cells of the middle rings form three 

 ciliary bands. The anal group of small cells produces a tuft of sensory cilia. 

 Under the integument of the larval mantle, the development of the defmitive 

 mollusk precedes (Figure 33). During metamorphosis the larval mantle is 

 discarded. 



Jagersten (1972) considers this bell-shaped larva a secondarily modified 

 one that was lost by the trochophore of moUusks even before the veliger 

 appeared in evolution. Other authors (for example, Fioroni, 1971; Salvini- 

 Plawen, 1972, 1973; Starobogatov, 1979) consider the lecithotrophic larva of 

 Protobranchia to be the starting point for bivalves. In our opinion, Jagersten's 

 viewpoint, also supported by Ivanova-Kazas (1977), is more justifiable. 



Development that includes lecithotrophic larvae is observed in all 

 Protobranchia irrespective of place of habitation — the Arctic, Antarctic, trop- 

 ics and littoral or benthic zones. Development may or may not include a brief 

 pelagic stage. 



In addition to Protobranchia, development without a planktotrophic larva 

 is typical of the suborders Carditida and Lucinida and coincides with different 

 forms of brood care. Some young are attached with the egg envelopes to the 

 substrate (Astartidae), others are carried in the gills right up to the juvenile 

 stage (genus Cardita), and still others, after brooding in ctenidia, are carried 

 in a chamber that forms on the ventral side of the shell of the female {Milneria 

 kelseyi and Thecalia concamerata) (Dall, 1903; Yonge, 1969). Development 

 without a planktotrophic larva is also typical of the order Pholadomyida (su- 

 perfamilies Myochamoidea, Clavigelloidea, Pandoroidea, Thracioidea). Thus, 

 in Pandoroidea a swimming larva emerges from the envelope after completing 

 metamorphosis within a few days (Pelseneer, 1911; Allen, 1961). A tendency 



