HABITS OF LARVAE 



481 



arisen from its edge. In this stage the larva leaves its mother's 

 shell and swims out into the world of water to look for a suit- 

 able place on which to settle down. This is the only stage in the 

 life history of a Brachiopod when the animal is locomotor, and can 

 serve to spread- its species. The extreme minuteness of the larva 

 and the short time it spends in this motile condition probably 

 accounts for the fact that Brachiopods are extremely localised. 

 Where they do occur they are found in great nund)ers, rocks 

 being often almost covered with them, but they are not li nuid over 

 large areas. When viewed under a microscope the lar^'ae seem 



Fig. 318. —Young larva of 

 Cistdla iieupolitana , 

 sliowiug three seg- 

 ments, two eye spots, 

 and two bundles of 

 setne. (After Kowa- 

 levsky. ) 



Fig. .319. — Full- -rown larva of Cis- 

 tdla neapoiUuna, witli unilirella- 

 shaped bead, ciliated. (After 

 Kowalevskv. ) 



to be moving with surprising rapidity, but judging from the 

 analogy of other forms, it seems doubtful if they swim a yard in 

 an lioni'. 



l-'requeutly the larva stands on its head for some time, as if 

 investigating the nature of the rocks on which it may settle : it 

 is extremely contractile, turning its head from time to time, 

 anil seldom retaining the same outline for any length of time ; the 

 setae are protruded, and at times stick out in every direction ; 

 they are possibly defensive in function. When fully stretched 

 out the larva is about -^ nnn. long, but it frequently shortens its 

 VOL. Ill 2 I 



