ON THE DEVELOPMENT OF LINGULA ANATINA. 41 



the mantle cavity. When the larvae are taken out of the plank- 

 ton sediments and placed in a spacious vessel of fresh sea water, 

 they first begin to swim about in their own peculiar manner 

 (PL VI., Fig. 88, a. b.). It reminds one of the figure of the 

 swimming Discinisca-larva given by Fritz Müller ('60), but 

 there are some differences. From between the wide gaping shells 

 the cirri are gracefully protruded out of the shell to almost two 

 thirds of their length. All the cirri are directed anteriorly with 

 a gentle curvature outward ; on the ventral or dorsal aspect there- 

 fore the cirri look like the ribs of a fan. In this way the 

 larvae swim very slowly, keeping their body axis nearly always 

 vertical. Sometimes they stand on the widely expanded arm- 

 apparatus upon the bottom of the vessel as if they were looking 

 for food. During the act of swimming the breadth of the body 

 proper is much reduced. 



Swimming larvae, when even slightly alarmed, retract their 

 cirri very rapidly into the mantle cavity, and sink to the bottom. 

 In this position they remain for a considerable length of time. 

 Larvae which give evidence of not having fed for a time are 

 sluggish and do not draw in their cirri, even when roughly 

 disturbed. They appear to have almost lost contractility. Such 

 larvae, therefore, can be brought upon a slide without danger of 

 causing the retraction of their cirri. 



Diatoms and other unicellular algae constitute the chief food 

 of the larvae. Among diatoms the genus with drum-like test 

 ( Ethmodiscus) predominates. 



Before giving detailed anatomy of the free-swimming larvae 

 a comparison of the figures of larvae, respectively with 5 p. c. 

 (PI. V. Figs. 77, 78.), 6 p. c. (PI. V., Fig. 79.), 7 p. c. (PI. VI., 



