42 



LAMELLIBKANCHIA. 



.as a hollow outgrowth of the ectoderm into which a great mass of 

 mesoderm -cells find their why. The gills arise in its neighbourhood 

 •either as two ectodermal ridges one on either side of the body at the 

 point where the inner lamella of the mantle-fold is continuous with 

 the body-epithelium {Teredo and Gyclas), or else at the same spot as 



a single row 7 of papillae {Mytilus, Dreisse.nsia 

 and the Unionidae, and, according to Jackson, 

 in Ostreri). 



In those stages of Gyclas and Pisidium 

 which must be regarded as the equivalents 

 nt' the Trochoj/horv, the foot has already 

 attained a considerable size. It occupies 

 the whole of the ventral surface between 

 the mouth and anus in the form of a massive 

 projection of the ectoderm (Fig. 19/). At 

 a later stage it extends in length and, in form 

 and position, shows the same relation to the 

 rest of the body as it does in the adult 

 (Fig. 21). 



In the free-swimming larva, the foot is 



already of considerable size. Although at 



first merely a truncated structure projecting 



only slightly below the shell, it soon grows in length, and can be 



protruded far beyond the shell when it appeal's to make vermiform 



movements and to function as a tactile organ (Fig. 20). 



Fig. 20. — Older larva of 

 /hrissnisii'i jiiih/iiinr/ihii 

 (original). /, foot; m, 

 mouth ; s, shell ; v, 

 velum. 



At this stage, therefore, the larva, besides its provisional locomotor}- organ, 

 the velum, also possesses the locomotor}' organ of the adult Lamellibranch. 

 Further, the foot usually, as far as we can judge from Dreissensia, is retracted 

 while the larva (which is still very active) swims about ; in this respect Fig. 

 20, which depicts the foot as extended in a larva with an expanded velum, 

 is not quite true to nature. Such a condition does, however, occasionally 

 occur when a larva has just extended the velum and is beginning to retract 

 the foot which was protruded beyond the shell as a sensory organ. 



The surface of the foot, at the Trochophore stage as well as later, 

 is covered with fine cilia. At the posterior upper boundary of this 

 ciliated area, in Gyclas, a pit-like depression of the ectoderm is found 

 on each side of the middle line, lying exactly above the mass of cells 

 from which the pedal ganglion develops (Figs. 19 and 21). This is 

 the paired rudiment of the byssal gland. 



